Born in Vilnius, Šiškevičius has competed as a professional since the start of the 2011 season, as the La Pomme Marseille team he had been a member successfully became a Continental team.

[4] Šiškevičius won the Volta ao Alentejo in 2011 – as well as a stage during the event[5][6] – before winning the Grand Prix de la Somme in September 2012.

[7] Šiškevičius returned to Team La Pomme Marseille 13 for the 2014 season,[8][9] after Sojasun folded at the end of 2013.

At the 2018 Paris–Roubaix, Šiškevičius finished the race over an hour after the winner, Peter Sagan.

He arrived so far outside of the time limit that when he arrived at the finish at the velodrome, the gates had already been closed, however, they were opened so that he could complete a ceremonious lap, despite his result still being marked as DNF.