LONDON, Nov. 7 (Xinhua) -- Defending champion Novak Djokovic of Serbia beat Juan Martin Del Potro to reach the semifinals of the ATP World Tour Finals here on Thursday.

Djokovic, who defeated Roger Federer in the first group match, joined world number one Rafael Nadal in the last four with a 6-3, 3-6, 6-3 victory over the Argentine.

It also extended his winning streak to 19 matches since losing to Nadal in the US Open final in September.

Del Potro, the fourth seed, and 7th-ranked Federer will fight for the second semifinal berth from Group B on Saturday.

Federer beat eighth seed Richard Gasquet of France 6-4, 6-3 in the day's other match.