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

LONDON — 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.

The 32-year-old 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.

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.