Summary Juan Martin del Potro clinched the last spot in the semifinals of the ATP finals on Saturday.
He has beaten already qualified Roger Federer 7-6 (3), 4-6, 6-3 in their final round-robin match.
It was Federer s first loss in the indoor tournament since 2009, ending a 12-match winning streak. He went undefeated in winning the last two titles and had won his first two group-stage matches this year.
Del Potro s win also leaves David Ferrer and Janko Tipsarevic with nothing but prize money to play for in their final group-stage match as both players are eliminated. However, that result will still decide whether Federer or Del Potro finishes top of Group B and will therefore impact the semifinal matchups at the season-ending event. The winner of the group will face Andy Murray on Sunday, while the second-place finisher has to take on top-ranked Novak Djokovic.
Del Potro, who also defeated Federer in the final in Basel last month, converted his only break point of the match in the opening game of the deciding set and then held his own serve comfortably to wrap up the win at London s O2 Arena.
He converted his first match point with a forehand passing shot that Federer could only return into the net with a lunging backhand volley.
Both players finished the group stage with 2-1 records.
While Federer had already secured a berth in the semifinals, he looked the more likely to take control of the match in the first set. However, he failed to convert three break points when leading 4-3, and Del Potro eventually held serve after a point where a call on the same shot was overturned twice.
A backhand from Federer was called just wide by the line judge, only for chair umpire Lars Graff to declare that the ball was in. Del Potro then challenged Graff s correction, and a Hawk-Eye replay showed that the ball just missed the line.
Federer, who had never looked threatened on his serve, then committed a series of uncharacteristic mistakes in the tiebreaker as Del Potro won six straight points to take a 6-1 lead. He then converted his third break point when Federer s backhand return sailed wide.
Federer bounced right back, breaking the Argentine in the next game. He then lost just five points on his serve the rest of the second set to level the match.
But in the third set, it was Del Potro s turn to break in the first game, and Federer never looked likely to muster another comeback.
