Sissis then chased his Grand Prix dream and went on to ride in the 2010 Red Bull MotoGP Rookie Cup. A best placing of 6th in the first race of Round 3 at Assen saw him finish 13th in the championship. He again rode in the championship in 2011, scoring three wins (Jerez race 2, Assen race 2 and Mugello race 1) and finished 2nd in the championship, only 9 points behind Italian Lorenzo Baldassarri.

In 2012, Sissis has joined the Red Bull KTM Ajo team for the Moto3 World Championship, replacing the 125cc World Championship. After qualifying 9th for the first race at the Losail International Circuit in Qatar, Sissis got a good start and was 2nd on the first lap before slowly dropping back and eventually finishing 7th.

The high point in Sissis’ racing career to date was his third-place finish at his home Grand Prix at Philip Island on 28 October 2012. Sissis led home a competitive pack behind the two leaders, his position only secured in the final seconds of the race, by only 0.054 seconds from fourth placed Alex Rins. This result was his only Grand Prix podium.

In December 2014, Sissis decided on a return to speedway, signing to ride in 2015 for Sheffield Tigers in the British Premier League.

In his return to speedway, Sissis finished 10th in the 2014/15 South Australian Solo Championship held at the Gillman Speedway in Adelaide on 28 December 2014.