Robin Soderling repeated his French Open heroics with a 6-4 6-4 victory over Rafael Nadal at the ATP World Tour Finals in London on Monday.
Soderling became the first man to beat the four-time champion at Roland Garros and with this result he could have scuppered Nadal's chances of overtaking Roger Federer to become the year-end world No.1.
The Swede went on to reach the final in Paris and has since climbed into the world's top 10, and he showed he could be a contender for the prestigious end-of-season crown at the O2 Arena with a display of power and nerve in the Group B opener.
Soderling's victory over Nadal at Roland Garros was arguably the most surprising result of the season, the Swede having lost his three previous matches to the world No.2.
There is no love lost between the pair and Nadal would have been desperate to avenge that painful defeat but things began badly as the second seed dropped his opening service game.
The fast indoor surface was always likely to suit Soderling - a late replacement for the injured Andy Roddick - more than Nadal, and he peppered the lines in the opening stages.
The Spaniard is a natural-born fighter, though, and a more aggressive approach did the trick as he broke back for 2-3.
But that did not shake Soderling's belief and a netted Nadal forehand gave the 25-year-old a set point on his opponent's serve, which he took thanks to an uncharacteristic unforced error from the Australian Open champion.
Soderling's huge serve was the bedrock of his first-set success but it cost him in the third game of the second as two double faults paved the way for Nadal to forge ahead.
However, the Swede simply upped the power level and forced three break-back points - taking the final one courtesy of a successful Hawk-Eye challenge.
A titanic fifth game finally went the way of Soderling and, although there were flashes of Nadal brilliance - notably some stunning passing shots - they were too infrequent to really put the underdog in trouble.
And, in a repeat of the opening set, the increasingly-frustrated Majorcan found himself staring down the barrel in the 10th game - but this time a break point for Soderling was match point.
Nadal saved one but the second chance was gleefully gobbled up by the Swede courtesy of a wayward Nadal backhand.
Defending champion Novak Djokovic produced a gutsy performance to clinch a 3-6 6-4 7-5 victory over Nikolay Davydenko in the other Group B clash.
It looked like another upset was on the cards when Davydenko took the first set but the world No.3 dug deep to come through in a match lasting nearly three hours.
Djokovic came into the tournament as arguably the man to beat after back-to-back titles in Basle and Paris. Indeed, his only defeat since the US Open came in the semi-finals of the Shanghai Masters last month - to Davydenko, who went on to beat Nadal in the final.
This was also a repeat of last year's final - again played in Shanghai - where the Serbian prevailed in straight sets.
The opening exchanges were predictably tight, both players looking at home on the indoor surface, but it was the ultra-consistent Russian who broke through first in the fifth game.
Suddenly Djokovic was making errors, and a double fault and wild forehand put