Novak Djokovic beats Juan Martin del Potro, reaches semifinals at ATP Finals

Novak Djokovic's victory over Juan Martin del Potro was his 19th-straight win. (Julian Finney/Getty Images) LONDON -- Novak Djokovic defeated Juan Martin del

Novak Djokovic's victory over Juan Martin del Potro was his 19th-straight win. (Julian Finney/Getty Images)

LONDON -- Novak Djokovic defeated Juan Martin del Potro 6-3, 3-6, 6-3 on Thursday to secure his spot in the ATP World Tour Finals and extend his winning streak to 19 matches.

The winner of del Potro's match against Roger Federer on Saturday in the last round of group play will advance to the semifinals as the second qualifier from Group B. Djokovic will play Richard Gasquet on Saturday.

Like he's been doing all fall, Djokovic showed great poise under pressure to earn his third straight win over del Potro. After dropping the second set in 35 minutes, Djokovic was down two break points early in the third set. He saved the first with an ace and the second by attacking the net, forcing del Potro to send a massive forehand wide. Del Potro was doomed by a poor service game in the sixth game, giving Djokovic the break at love.

At times this season, Djokovic struggled to maintain his focus during key moments of big matches. However, that seems to be a thing of the past since the U.S. Open.

"I'm playing the best tennis I think I played throughout the whole season," he said. "That's something that I take as a positive for this moment and obviously next season. I'm definitely feeling confident, more confident than I was a few months ago."

The fall swing can be a grind for other players, but Djokovic has excelled. In the last two seasons he has gone 34-1 after the U.S. Open.

"I know most of the player are exhausted physically, mentally, emotionally, after a long season," he said. "I try to find that as an opportunity for me to get extra motivated, push myself to the limit and deliver the goods when needed."