Janković was born in Belgrade, Socialist Federal Republic of Yugoslavia, as the third child of Veselin and Snežana, both economists. She has two brothers, Marko and Stefan. She is a student at the Megatrend University in Belgrade, studying economics. However, she has put her course of study on indefinite hold as she continues to pursue her tennis career. Janković learned her first tennis skills at the Tennis Club ‘Red Star’. As a 9 ⁄2-year-old she was introduced to tennis by her elder brother and fitness coach Marko. She was later trained at the Tennis Academy of Nick Bollettieri. As a junior, she won the 2001 Australian Open and reached junior no. 1 in the world. In 2001, she started to play on the WTA Tour; she reached the second round at her first tournament at the Indian Wells Masters.

In October 2003, Janković entered the top 100 at No. 90 for the first time, after winning her first ITF title in Dubai. Three months later, Janković garnered her first top-10 win against Elena Dementieva in the first round of the 2004 Australian Open. In May, Janković won her first WTA title, a Tier V event, in Budapest, defeating Martina Suchá in the final. Following her win in Budapest, she reached No. 51 in the world. Elsewhere in her 2004 season, she defeated top-20 players Nadia Petrova (twice), Vera Zvonareva, Patty Schnyder, Elena Dementieva and Paola Suárez. Janković finished 2004 ranked no. 28 in the world.

In 2004, they played their first match in San Diego; Serena won in three sets. The second match was at the 2005 Dubai Tennis Championships semifinal, with Janković causing an upset (Serena was world No. 4). When Janković was leading 6–0, 4–3, Serena retired. She is one of nine players who won a 6–0 set against Serena: Alexia Dechaume-Balleret, Mary Joe Fernández, Justine Henin, Patty Schnyder, Venus Williams, Anabel Medina Garrigues, Simona Halep) and Johanna Konta.

Jelena Janković and Vera Zvonareva have met 14 times with Zvonareva leading their head-to-head 8–6. Their first meeting was in 2005 with Zvonareva winning a second round match in Rome.

At Wimbledon Championships, she lost in the second round of singles to Vesna Dolonc but made it to the quarterfinal of doubles (in pair with Mirjana Lučić-Baroni) where they lost to eventual champions Hsieh Su-wei and Peng Shuai. Janković kicked off her US hard-court season in Carlsbad, where she was upset in the second round by Urszula Radwańska in three sets. She also competed in the doubles tournament with Katarina Srebotnik and lost in the semifinals. This propelled her to a career high in doubles, No. 38, seven years after her last career high. She then competed at Toronto, and lost to eventual finalist Sorana Cîrstea in the third round. Janković again partnered Srebotnik in the doubles. The unseeded pair won the tournament. This marks Janković’s second career doubles title, her 1st title in Birmingham in 2006. This result will further increase her doubles ranking to a career high, No. 25. At the Western & Southern Open, Janković defeated four top-25 players en route to the semifinals: Sabine Lisicki, Ekaterina Makarova, Sloane Stephens, and Roberta Vinci. She then lost to world No. 2, Victoria Azarenka in three tough sets. This result means that Janković clinches the 9th seed at the US Open, after the retirement of Marion Bartoli and withdrawal of Maria Sharapova. And as a top 10 seed at a Grand Slam for the first time since 2011 French Open, Janković made it to the round of 16 for the first time at the US Open since her run to the final in 2008 after beating Madison Keys, Alisa Kleybanova, and Kurumi Nara, all in straight sets. She was defeated in the fourth round by a peak form Li Na. As a result, Janković was back into the top 10 singles rankings for the first time since June 2011.

They have met three times in Grand Slam tournaments. Janković caused an upset at Wimbledon in 2006 where she defeated the defending champion in the third round, winning in three sets. They met twice at the Majors in 2007, Janković winning at the French Open, and Williams winning at the US Open in 2007. Both matches required a third set.

Janković won at the LA Women’s Tennis Championships in 2006 in straight sets, and Serena won at the Australian Open in 2007. At the Australian Open in 2008 quarterfinal, Janković won in two sets.

Janković has reached the singles final of the US Open and the singles semifinals of the Australian Open and the French Open. In 2007, she became the first female Serbian player to win a Grand Slam title when she won the Wimbledon mixed doubles title with British partner Jamie Murray.

A prominent rivalry in women’s tennis was the one between Janković and fellow Fed Cup teammate and Serbian compatriot Ana Ivanovic. They have met 12 times, with Ivanovic leading 9–3 overall. Seven of their eleven meetings have come at important stages of tournaments, while two have come at Grand Slam tournaments; Janković trails in both categories 1–6 and 0–2 respectively. Both players were touted as future top players following their breakthrough in 2007 and 2008, however, both have since underperformed on the WTA Tour and dropped out of the Top 10 in contrasting circumstances. Matches between Janković and Ivanovic are often given the nickname “The Battle of Serbia”. Since the end of 2008, both have struggled at the Grand Slams, with Janković only being able to reach the semi-finals of the 2010 French Open and the quarter-finals of the 2013 French Open and Ivanovic the quarter-finals of the 2012 US Open and 2014 Australian Open, and the semi-finals of the 2015 French Open.

On 5 December 2007, Janković became a UNICEF National Ambassador for Serbia, for Children’s Fund. “I am happy to have become a UNICEF ambassador for Serbia. This is a great honour for me and I will try to justify the role that has been given to me”, she said. Janković is the second Serbian tennis star to have volunteered to help promote the rights of children and collect funds for UNICEF after Ana Ivanovic became an ambassador in September.

At Wimbledon, Janković injured her knee during her third-round match against Caroline Wozniacki. Although she won that match, it visibly hindered her during her fourth-round loss to Tamarine Tanasugarn. Janković would have become the world no. 1 had she reached the semifinals of the tournament. However, she lost to Tamarine Tanasugarn in the fourth round. Janković squandered two further opportunities to immediately take the world no. 1 position with losses in the semifinals of the East West Bank Classic in Los Angeles to Dinara Safina and the quarterfinals of the Rogers Cup in Montreal, to Dominika Cibulková. Janković did eventually replace Ivanovic as the world No. 1 on 11 August 2008. She was the 18th woman to have been ranked world No. 1 by the Women’s Tennis Association, but the first woman to have done so without ever having reached a Grand Slam final and only the third woman (the others being Amélie Mauresmo and Kim Clijsters) to have become world No. 1 without first winning a Grand Slam title.

Janković became the world no. 1 again during the fall in the midst of winning three consecutive tournaments. At the China Open, Janković defeated Svetlana Kuznetsova in two sets to win the title. The following week, Janković played in the Porsche Tennis Grand Prix in Stuttgart, where she defeated Venus Williams in the semifinals and Nadia Petrova in the final. In the Kremlin Cup in Moscow, Janković defeated defending champion Dementieva in the semifinals before triumphing against Vera Zvonareva in the final for her third title in three weeks, the first time for a player on the WTA Tour to do so since 2005. As the top seed at the year-end 2008 WTA Tour Championships held in Doha, Janković won two of her three round-robin matches, including only the second win of her career over Ivanovic. However, she lost in the semifinals, to eventual champion Venus Williams and ended the year as world No. 1. In all, Janković lost to eventual champions at six tournaments (including three Grand Slam tournaments) during the season. Janković was later named the ITF World Champion for her performance in 2008. She won four titles during the season, the most of anyone on the tour along with Serena Williams and Safina.

They met six times in 2008 with Janković winning four of the six meetings, including three consecutive times in three consecutive tournaments during the fall. The third match in this streak was the final of the Kremlin Cup, which was the third consecutive title won by Janković in a stretch which also included titles in Beijing and Stuttgart. Janković had defeated Zvonareva on her way to the two titles which preceded the Kremlin Cup. However, Zvonareva would defeat Janković at the WTA Tour Championships later that year; this would be the start of an ongoing five-match winning streak against Janković.

At the 2008 Sony Ericsson Open, Miami, they met in a final for the first time. Serena won in three sets. They played again in the final, and again in USA – at the US Open in 2008. Serena won in two sets. In the second set Janković had 5–3 and three consecutive set points on Serena serve. Later, when she was serving for the set at 5–4, she missed one more set point. However, Serena saved all set points and won four games in a row for US Open title.

She was ranked world No. 1 for 17 consecutive weeks until she was overtaken by Serena Williams on 2 February 2009. She was the year-end world No. 1 in 2008, the first player in the history of the WTA Tour to do this without winning a Grand Slam title.

At the Australian Open, Janković, seeded eighth, lost to 31st seed Alona Bondarenko in the third round. Next, Janković represented Serbia in the 2010 Fed Cup. In her first match, she rallied from 4–6, 1–4 down to defeat Alisa Kleybanova, then defeated reigning French Open champion Svetlana Kuznetsova. In the deciding doubles match, she partnered with Ana Ivanovic but fell to Kuznetsova and Kleybanova. Her next scheduled tournament was the Dubai Tennis Championships where she lost to Vera Zvonareva in the third round. She traveled to Indian Wells for the BNP Paribas Open. At the tournament, she was seeded sixth and defeated Caroline Wozniacki to take her 12th title. Janković’s next tournament was the Sony Ericsson Open, where she was seeded seventh. In the fourth round, she lost to Samantha Stosur, ending her eight-match winning streak.

The pair were involved in a long-running feud in early 2010, arising from Ivanovic’s decision not to play in Serbia’s Fed Cup World Group play-off against Slovakia due to her poor form at the time. They met in a controversial encounter at the Madrid Premier Mandatory tournament in May that year, in which Janković appeared to criticise and imitate Ivanovic’s fist-pumping habit following a three-set victory. It was only Janković’s third (and most recent to date) victory against Ivanovic, after previously winning in 2006 and 2008.

Their only meeting at a Grand Slam tournament was at Wimbledon in 2010, in the tournament’s fourth round. Zvonareva won, reaching her first quarter-finals of the grass court Major (and eventually went on to reach the final), after Janković retired whilst trailing in the second set.

At Indian Wells in 2011, Janković was the defending champion, but Ivanovic defeated her in the fourth round, ending her title defence. That would be their last meeting in a WTA Tour match until the 2013 Australian Open, where Ivanovic prevailed in the third round. Their most recent meeting was in the semi-finals of the Porsche Tennis Grand Prix in 2014 with Janković losing again in straight sets.

Their most recent meeting was at the Qatar Ladies Open in 2011 with Zvonareva winning in three sets. It was her fifth straight victory over Janković, dating back to 2008.

Janković began 2012 at the Brisbane International, losing to Francesca Schiavone in the quarterfinals. She then lost in the second round of the Apia International to third seed Victoria Azarenka. At the Australian Open, Janković was defeated by Caroline Wozniacki in the fourth round.

Janković is currently sponsored by Italian sportswear company Fila. Previously, Janković had endorsed Anta and Reebok sportswear, and had her own line with them for her tournament wear. Janković also has an endorsement with Prince Sports: in her early career until 2007, she played with the original Prince O3 Red; from 2008 to 2010 she used the Prince O3 SpeedPort Pro White; from 2011 to 2013 Janković played with different types of racquets and in 2012 and 2013 she chose blacked out ones; since January 2014 Janković has been playing with the Prince O3 Red LS.

At the final of the 2013 Family Circle Cup, Charleston, Serena won in three sets. Again in 2013, at China Open, they met in the final and Serena won in two sets. At WTA Championships Serena won their semifinal match after losing second set convincingly, 6–2.

Janković began 2015 at the Brisbane International. Seeded sixth, she lost in the first round to Ajla Tomljanovic. She then admitted that she had recently thought to end her career because of a back injury. Also, she added that she lost a lot of muscle mass, had not done any activity in past two months and that she needs time to get back in normal form. Bad results continued in January, including a first-round loss at the Australian Open to Timea Bacsinszky. She had never lost her opener in Melbourne prior to this.

Due to her back surgery in October 2017, Janković missed the entire 2018 and 2019 seasons. She also underwent eye surgery in May 2018, further delaying any comeback plans. For the first time since 2001, she was unranked in the WTA singles rankings.

They have also met twice at the WTA Tour Championships, both times in the semifinals with Williams winning in three sets on both occasions. Their most recent meeting transpired in the 2017 BNP Paribas Open in Indian Wells, California. Janković appeared on her way to victory in this second round meeting at 6–1, 4–1 up before Williams came racing back to win a 2nd set tiebreak and force a decider, which she proceeded to win 6–1 en route to the quarterfinals.