Söderling beat Djokovic 7-6 (7/5), 6-1 on Wednesday to reach the semi-finals.
Söderling is only competing in this prestigious season finale because of an injury to Andy Roddick, but his victory over Rafael Nadal in his opening match made it clear he is determined to take advantage of his good fortune.
The 25-year-old, ranked ninth in the world, underlined the point by making Djokovic the latest victim of his impressive form this season.
Söderling, who ended Nadal’s four-year reign as French Open champion with a shock win en route to the final earlier this year, had lost his five past meetings with Djokovic, with his most recent defeat to the Serb coming just two weeks ago in the Paris Masters.
That run came to an end in emphatic fashion in this Group B clash.
After a marathon three-set victory over Nikolay Davydenko that didn’t finish until close to midnight on Monday, Djokovic must have been relieved to see his match scheduled in the more sociable early slot.
Whatever time of day the world number three plays, Djokovic has been in supreme form of late, winning 19 of his last 20 matches.
But Djokovic never found any rhythm against the ruthlessly efficient Swede.
A hard-fought set looked to have swung Söderling’s way when Djokovic gifted him three set-points at 4-5.
Djokovic, defending superbly at the back of the court, clawed back all three and the set went to a tie-break.
Again it was Söderling who took control at the crucial moment as he hit a perfect volley to earn two more set points. His nerve held this time and he converted the second set point to move into the lead.
The Swede is widely regarded as one of the world’s best indoor players and he kept the pressure on Djokovic in the second set.
Djokovic hardly helped himself though, attempting a drop-shot on break point that he flunked completely and gave Söderling a 2-1 lead.
That was enough to send the Serb into something of a meltdown. He had already been gesturing angrily towards his coaching team in between points and his composure disappeared completely as Söderling broke again to finish the contest.