Argentina's Juan Martin del Potro defeated Roger Federer 7-6 (7-3), 4-6, 6-3 here Saturday at the Barclays ATP World Tour Finals to advance to the semifinals of this year-end event for the second time.
The seventh-ranked Del Potro staved off three break points in the eighth game and ended up winning the first set in a tiebreaker, benefiting from a series of unforced errors by the Swiss world No. 2.
But Federer quickly turned the momentum around by breaking Del Potro's serve at the start of the second set and then controlling his service games by limiting his unforced errors.
In the third set, however, it was the Argentine who got the early break and then held on for the win in the players' final Group B round-robin match.
"It's a surprise for me now. I just was thinking about my game, trying to beat him and go to the semis. Here I am feeling confidence with my game," Del Potro said in the post-match press conference.
"My serve is working really well. Maybe the surface helps to play short rallies, to be aggressive all the time. That, for my game, is a very good advantage."
Del Potro's victory, which eliminated Spain's David Ferrer, marked just the second time in Federer's career that an opponent had defeated him in two consecutive indoor tournaments.
The Argentine also topped Federer last month in the final of the Swiss Indoors Basel tournament.
Del Potro reached the final of this event in 2009 but lost to Russia's Nikolay Davydenko 6-3, 6-4, while Federer has won the tournament a record six times.
The eight-man Barclays ATP World Tour Finals is the biggest tournament on the men's tennis calendar after the four Grand Slams.