Roger Federer fought back Saturday to reach the semifinals in the ATP World Tour Finals.

On the back of a poor season, the 17-time Grand Slam winner overcame a 3-0 deficit in the third set to beat Juan Martin del Potro 4-6, 7-6 (2), 7-5 to reach the semifinals for the 11th time.

The Swiss earned a chance to prove that - despite his aging legs, aching back and recent lack of consistency - he can still beat rival Rafael Nadal, his semifinal opponent.

''I just have to have the right mindset to give it one last go, maybe play with a little less pressure than I have in previous matches with him,'' Federer said. ''I just feel like I need to look at it more as being an underdog a little bit because of circumstances, because of my year, because of his year. Maybe that free swinging is what I kind of need to do a little bit more tomorrow.''

The 32-year-old Federer has endured a tough year and the elite event is his last chance to win a big title in 2013. Federer has just four victories against top-10 opponents in 2013 and has not beaten Nadal since 2012. The top-ranked Spaniard leads 21-10 in their head-to-head series, but Federer is 4-0 on indoor hard courts, having secured all four wins at the ATP Finals.

Nadal, who returned to tennis in February following a seven-month layoff due to a knee injury, clinched the year-end No. 1 spot this week in a season in which he won the French Open and the U.S. Open. In London, the Spaniard qualified from his group unbeaten.

''We know each other that well. He's going to do exactly what he needs to do, I'm going to try to do what I need to do,'' Federer said. ''Hopefully it's going to match up good for me. If not, he deserves it. He's had a wonderful season.''

Novak Djokovic, who beat Richard Gasquet 7-6 (6), 4-6, 6-3 in his final round-robin match to extend his winning streak to 20 matches since his loss to Nadal in the U.S. Open final in September, will face Stanislas Wawrinka in the other semifinal.

Federer won only the grass-court event at Halle, Germany, in June. He regularly lost to unseeded opponents and secured his qualification for the Finals at the last minute.

''The goal is always to win every tournament. But I think I had to be slightly more realistic in recent weeks and months,'' Federer said. ''I knew that sometimes just getting a few rounds into the bag was positive.''

Both Federer and del Potro needed a win to progress from Group B and treated the crowd to a tense and entertaining back-and-forth match. Federer trailed in all three sets.

Only once in 12 appearances has Federer failed to advance to the last four at the season-ending tournament for the top eight players.

He struggled to return del Potro's big serve in the first set and committed seven forehand mistakes as his rival opened up a 5-1 lead.

Capitalizing on more Federer errors, del Potro broke again in the third game of the second set. The Argentine looked set for an easy win but failed to build on the momentum, double-faulting as Federer leveled at 3-3.

Federer then gradually took the upper hand with a brilliant display at the net and played a nearly prefect tiebreaker as del Potro crumbled. Federer clinched the set with an ace.

But Federer again started sluggishly in the decider, allowing del Potro to break as he hit two consecutive weak forehands.

But the blip was temporary and, once again, his rival could not take advantage of his inconsistency. At 3-1, del Potro saved a break point but fluffed a forehand on the second one, bringing Federer back into the match.

Hitting more confidently toward the end, Federer broke to go up 6-5 and saved a break point in the next game before closing out the match with his 10th ace.

''I think I got two chances to win the match,'' del Potro said. ''But in the end you have to be focused and to find the winners. I made the mistakes, and he was there really focused to take the chance.'' The mark of the champions.

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