Federer, 32, came back from a set down, and then a break down in each of the final two sets, to beat Argentina’s Juan Martin del Potro 4-6, 7-6 (7-2), 7-5 at London’s O2 indoor arena.
The win gave the Swiss his 11th semifinal in 12 appearances at the ATP World Tour Finals, a tournament which is reserved for the top eight men. He’ll face top-ranked Rafael Nadal, a player he hasn’t defeated since March 2012, tomorrow for a place in the final.
“I just feel like I need to look at it more as being an underdog a little bit,” Federer said of his match with the Spaniard. “Maybe that free swinging is what I kind of need to do a little bit more tomorrow.”
Djokovic completed his 100 percent record in group play with a 7-6 (8-6), 4-6, 6-3 win over eliminated Richard Gasquet of France. The Serb had already guaranteed his semifinal spot with earlier victories against Federer and del Potro.
Djokovic told reporters it was a mental challenge playing a meaningless match of more than two hours with the semifinal on tap tomorrow. He made 45 unforced errors to 33 for Gasquet.
The No. 2-ranked player and defending champion, Djokovic next plays Stanislas Wawrinka. He’s won 14 of 16 previous meetings with the Swiss and goes into the match riding a 20-match winning streak.
Federer, a 17-time Grand Slam singles champion has a 10-21 career record against Nadal, but is 4-0 indoors. He’s won the season-ending event a record six times, while 13-time Grand Slam winner Nadal’s best finish is a loss to Federer in the 2010 final.
In the doubles, third seeds Ivan Dodig and Marcelo Melo, who’d already claimed a semifinal berth, beat eliminated Aisam-Ul-Haq Qureshi and Jean-Julien Rojer 7-5, 3-6, 11-9. Top seeds Bob and Mike Bryan of the U.S. beat Mariusz Fyrstenberg and Marcin Matkowski 4-6, 6-3, 10-5 to reach the last four.
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