In an upset that left the London crowd stunned, No. 6-seeded Juan Martin del Potro defeated No. 2-seeded Roger Federer in three sets (7-6, 4-6, 6-3) to advance to the semifinals at the 2012 ATP World Tour Finals.
Before a raucous crowd, the match was a thriller from the outset, as Federer and del Potro traded service points en route to a tiebreaker in the first set. Del Potro won that tiebreaker and seemed to gain control of the match, but in typical Federer fashion, he roared back to break his opponent in the second set and win 6-4.
However, Saturday just seemed to be 24-year-old del Potro's day. He dominated the third set, breaking Federer early and holding throughout to come away with a scintillating victory.
The loss snapped a 12-match winning streak at the World Tour Finals for Federer, who is also the two-time defending champion at the event.
Luckily for Federer, he still has a chance to capture the Group A title if David Ferrer defeats Janko Tipsarevic later on Saturday. The reeling Swiss star would then take on Andy Murray in the semifinals on Sunday to set up a matchup against either Novak Djokovic or del Potro in the finals.
Meanwhile, that scenario means del Potro's reward for defeating Federer on Saturday is a match against Djokovic, who has looked like an impenetrable force at the World Tour Finals. The 25-year-old Serbian somehow survived a brilliant match against Murray in round-robin play and heads into the semis with a 43-31 lead in games thus far.
If Tipsarevic wins, however, the reverse happens. Federer would then take on Djokovic in the semis, while del Potro would win the group and take on Murray.
Nevertheless, del Potro's victory on Saturday over Federer is once again proof that the young Argentinian can defeat any player in the world when he's at the top of his game. Just a brilliant performance in a thrilling match for del Potro.