Juan Martin Del Potro joined Roger Federer in the semi-finals of the Barclays ATP World Tour Finals after ending the defending champion's 12-match winning streak at London's O2 Arena.
Federer has won the title the last two years with a perfect record but knew he was already through after straight-sets wins in his first two group matches.
The incentive was therefore with Del Potro and he seized his chance, recovering quickly after dropping the second set to win 7-6 (7/3) 4-6 6-3.
The semi-final match-ups will not be decided until after the match between David Ferrer and Janko Tipsarevic, even though it is now a dead rubber.
Victory for Ferrer would mean it is Federer who plays Andy Murray, with Del Potro playing Novak Djokovic, while a Tipsarevic win would reverse the fixtures.
Federer had won 13 of his previous 16 meetings and six of seven this year with Del Potro, but the Argentinian is a player who has given him plenty of trouble.
As well as winning his only grand slam title with victory over Federer in the final of the US Open in 2009, Del Potro also led the Swiss by two sets in the French Open quarter-finals this year and pushed him to 19-17 in the third set at the Olympics.
The 24-year-old handed Federer his only other indoor defeat in the last two years in the final of his home-town tournament in Basle two weeks ago.