(CNN) -- Robin Soderling blew Group B at the ATP Tour Finals in London wide open with a shock straight sets victory over world number two Rafael Nadal on Monday.
In Monday's second match in Group B, defending champion Novak Djokovic saw off a determined challenge from Russian Nikolay Davydenko to register a three sets victory.
The in-form Serbian had to come from a set down to beat Davydenko 3-6 6-4 7-5 in a late night encounter lasting over two and a half hours.
After the opening two sets were shared, Paris Masters winner Djokovic gained an early break in the decider, but came under pressure on his own service as he saved break back points in a marathon sixth game, but then finally succumbed as he served for the match at 5-4, Davydenko finally leveling on his fifth break point.
But it proved a temporary reprieve as Djokovic brilliantly broke him back to love in the next game and then served out for victory, winning on his second match point as Davydenko hit long.
Soderling only made the elite eight-man tournament after Andy Roddick pulled out through injury but took his chance with both hands as he brushed aside Nadal 6-4 6-4 before a stunned crowd at the O2 Arena.
Soderling had beaten Nadal in the fourth round of the French Open on the way to losing in the final against Roger Federer.
After his shock defeat, the Spaniard took a prolonged break to recover from tendinitis in both knees.
He has failed to win a tournament since May, but went into the round-robin opener as favorite against an opponent making his first opponent in the finals.
But Soderling showed few signs of nerves with an immediate break of service to lead 3-0.
Nadal broke back and it looked headed for a tiebreaker until he wobbled in the 10th game on his own service, Soderling taking the opener as Nadal made two forehand errors from deuce.
The second set followed a similar pattern, with both players trading breaks, but Soderling looking the more solid.
Serving at 4-5, Nadal ran into trouble again, saving the first match point as Soderling hit long, but then making his 24th unforced error of the match to hand the Swede a momentous victory.
"I was very nervous at the start, but got off to a great start and that really helped me today," Soderling told Sky Sports.
Nadal can still qualify from one of the top two places in the group, but need results to favor him.
Tuesday sees world number one Federer battle home hope Andy Murray after both made winning starts in Group A.
Australian Open champion Nadal had hopes of overhauling Federer as world number one, but the defeat means his Swiss rival needs to win just one more match at the finals to make sure he retains the ranking.
Nadal played tribute to Soderling after his disappointing defeat.
"He is a big player in this surface, so it's difficult," he said.
"I didn't play really bad. But in the important moments, I didn't have this calm, this necessary calm, to win these matches against the best players of the world," he added.