In 2004, he played in the Thomas Cup with Chew Choon Eng. They gave a strong performance during the second doubles match against Flandy Limpele and Eng Hian of Indonesia in the quarter finals. However, after the tournament, the Badminton Association of Malaysia decided to partner him with Chan Chong Ming who previously partnered Choon Eng. Later, they won the bronze medal at the 2005 World Championships.

In 2006, he started to play with Tan Boon Heong. Together, they won the gold medal at the 2006 Doha Asian Games as an unseeded pair. En route to the finals, they defeated several top pairs including Markis Kido and Hendra Setiawan of Indonesia. They are the youngest ever men’s doubles pair to win an Asian Games gold medal at the age of 22 and 19 respectively. 2007 was the best year for Koo and Tan. They won several Super Series tournaments and climbed to the top of world rankings. They also won their first All England Super Series title after beating Chinese pair, Cai Yun and Fu Haifeng in straight games.

In 2009, the two won the bronze medal at the 2009 World Championships. At the 2010 BWF World Championships, the pair entered the semifinals after beating Korean rivals Lee Yong Dae and Jung Jae Sung. In the semifinals they defeated China’s Guo Zhendong and Xu Chen 21-14, 21-18. Tan and Koo became the first Malaysian pair to enter a World Championship final in 13 years. In the finals, they played China’s Cai Yun and Fu Haifeng but lost 21-18, 18-21, 14-21.

In 2010, they won their first title of the year in their home, Malaysia, as the world number 1 pair. They came in seeded number 1 in the All England but lost in the first round to Denmark former world champions Lars Paaske and Jonas Rasmussen.

In the 2010 BWF World Championships they beat the young Chinese pair of Chai Biao & Zhang Nan in the quarter-finals and later their arch rivals, the South Koreans Jung Jae Sung and Lee Yong Dae, in 3 sets to reach the semi-finals. After that, they beat another Chinese pair of Guo Zhendong & Xu Chen to reach their first ever finals in world championship. The only other Malaysian pair to reach that far before them were Yap Kim Hock and Cheah Soon Kit.

At the 2011 All England Super Series Premier, Boon Heong and Kien Keat defeated 2008 Olympic champions Markis Kido and Hendra Setiawan in the quarterfinals. They then defeated World champions Cai Yun and Fu Haifeng 21-11, 23-21. They lost to the Danes and then world number 1 Mathias Boe and Carsten Mogensen 21-15, 18-21, 18-21.

In 2012, they competed at the 2012 Olympics, reaching the semi-finals losing to the eventual gold medalists, and then losing in the bronze medal match to the Koreans. In 2013, they suffered a series of early round exits in 2013 and a three-year major title drought but managed to remain in the top 10 of the world rankings. In 2014,Koo parted with the Badminton Association of Malaysia and became a coach for the Granular Club of Thailand in early 2014. In August that year, Koo returned to play his last tournament with Tan at the 2014 BWF World Championships. Their supposedly last match together was in the third round where they lost to a Chinese Taipei pair with a score of 19-21 in the deciding game.

He began his studies at St. Michael’s Institution in Ipoh, Perak. Later, he received an offer to go to Bukit Jalil Sports School, where he completed his secondary school studies and sat for the SPM (Sijil Pelajaran Malaysia) examination. He married Audrey Tan Su Ven in 2013, and has a son, Dayson, who was born in 2014.

In 2015, Koo announced that he is coming out of retirement to qualify for the 2016 Rio Olympics with Tan before they call it quits for good. They were sponsored by Seri Mutiara Development Sdn Bhd and playing for an independent club. They achieved several breakthroughs this year, winning the Dutch Open and finishing as runners-up at the Thailand Open. They also made it to two Super Series quarterfinals in Australia and Korea.

In 2016, Koo and Tan managed to enter the top 15 of the world rankings. However, due to the new Olympic qualification requirement set by the BWF whereby each country can send two representatives for each event only if they are both in the top 8 of the world rankings in their discipline and if they are not then only the highest ranked representative will contest, Koo and Tan narrowly failed to qualify for the Olympics. By the time the qualification period had ended, the two were ranked world number 14, just one rank behind compatriots Goh V Shem and Tan Wee Kiong who were selected instead and went on to win the silver medal. In November, Koo reannounced his retirement from professional badminton, citing the Hong Kong Open as his last tournament.