Roger Federer (R) of Switzerland shakes hands with Juan Martin Del Potro of Argentina after winning his men's singles tennis match at the ATP World Tour Finals at the O2 Arena in London November 9, 2013. [Photo/Agencies]

LONDON - Sixth seed and six-time former champion Roger Federer battled back from a 0-3 deficit in the third set to beat fourth seed and 2009 runner-up Juan Martin del Potro to reach the semifinals at the ATP World Tour Finals here on Saturday.

The Swiss former No. 1 won 4-6, 7-6 (2), 7-5 to finish in second position in Group B for a clash against top seed and 2010 finalist Rafael Nadal on Sunday.

Second seed Novak Djokovic of Serbia, who had already qualified, beat Frenchman Richard Gasquet 7-6, 4-6, 6-3 in the other match to top Group B. The reigning champion will next face seventh seed Stanislas Wawrinka of Switzerland in the semifinals.

Nadal leads Federer 21-10 in their previous meetings and has won their past three matches.

"I was probably slightly angry more than thinking it's going to be over soon," said Federer, when asked about how he felt at 0-3 in the third set. "It's one of those moments today, because I kind of fought back the whole match -- the first, second set. Here we go again."

"I think I got two chances to win the match," said del Potro. "I broke his serve in the second set and in the third one. But he played great when I was up, and he deserved to come back in both sets. But at the end, when you have to be focus and find the winners, I made the mistakes and he was there really focused to take the chance. I think that was the key of the match."