Juan Martin Del Potro joined Roger Federer in the semi-finals of the Barclays ATP World Tour Finals after ending the Swiss's 12-match winning streak at the 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 Saturday night's 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, meanwhile, 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.
It was a very tight first set, with the only break points coming for Federer in the eighth game, but Del Potro saved all three.
And it was the Argentinian who dominated the tie-break, Federer choosing a bad moment to misfire with a series of poor forehands and backhands.
His response was emphatic, going on the offensive and winning the first eight points of the second set, and Del Potro did not come close to retrieving the break.
The sixth seed did strike first in the decider, though, taking a 2-0 lead when Federer miscued a backhand.
The champion was making a lot of errors and let out a cry of frustration midway through the set as he battled to turn things around.
There was no way back, though, and Del Potro sealed an impressive victory after exactly two hours when Federer netted a volley.