Defending champion Novak Djokovic will take on third seed Stan Wawrinka in the US Open final after both men won semi-finals in punishing conditions.

Djokovic beat 10th seed Gael Monfils 6-3 6-2 3-6 6-2 in New York, with the Frenchman adopting unusual tactics to try and counter the world number one.

Stifling humidity was still a factor as Wawrinka beat Japan's sixth seed Kei Nishikori 4-6 7-5 6-4 6-2.

"It's really humid, really hot. I knew it would be tough," said Wawrinka.

Djokovic, 29, had treatment on both shoulders during the first semi-final, while Monfils could barely move at times as conditions took their toll.

"Humid," is how the Serb described it. "I think that's all we were thinking about and feeling."

Monfils robustly rejected the notion that he did not give his all, saying: "I think I tried my best."

Djokovic will attempt to win his third US Open and 13th Grand Slam title on Sunday, while Wawrinka - who beat Djokovic in last year's French Open final - is going for a third major victory and first at Flushing Meadows.

Djokovic had spent only six hours and 24 minutes on court in reaching the semi-finals, thanks in part to one walkover and two retirements, and he was within sight of another swift contest against Monfils.

He led by two sets and a break as Monfils spent much of the first hour hitting the ball at half-pace, barely moving during, as well as between, points and at one stage limping theatrically.

However, he raced forward to collect a drop shot and break back in the third set, sparking a run of five straight games.

Djokovic's fortunes took a nosedive, first as he called for treatment to his left shoulder and then as three break-back points slipped by, and with them the set, prompting him to rip his shirt in frustration.

Monfils, 30, was back in the match but he was hardly fizzing with energy, calling for the trainer early in the fourth set, bending double at every opportunity and even drinking a Coca-Cola at one changeover.