Federer, seeking a 21st Grand Slam title and sixth US Open crown, comfortably dispatched Japan's Yoshihito Nishioka 6-2, 6-2, 6-4 on Arthur Ashe Stadium to cruise into the next round.

The Swiss star, who has not won the last Grand Slam of the year since 2008, hit 56 winners in a dominant performance.

Federer will play Frenchman Benoit Paire, a 7-6, 3-6, 7-5, 7-6 victor over Austrian Dennis Novak, in round two.

Serbian Djokovic, who is aiming for a 14th Grand Slam, struggled in the heat and humidity during his  6-3, 3-6, 6-4, 6-0 win over Hungary's Marton Fucsovics.

The 31-year-old asked for a sick bucket at one point during the match as temperatures soared to 38 degrees centigrade, while the humidity was over 50 per cent in the American city.

Both players were given a 10-minute heat break in between the third and fourth sets - the first time organisers of the tournament have implemented an extreme heat policy.

They later confirmed that five men had withdrawn from the singles event due to the heat, with some describing conditions as "dangerous".

Djokovic, seeded sixth, will face American Tennys Sandgren in the next round.

"I was praying that I got to feel better because I wasn't feeling great for most of the first three sets," Djokovic said.

"I want to thank the US Open for allowing us to have a 10-minute break after the third set.

"We both needed it."

In the women's draw, Australian Open champion Caroline Wozniacki of Denmark overcame Australian Sam Stosur 6-3, 6-2 to set up a meeting with Ukraine's Lesia Tsurenko.

Reigning Wimbledon champion Angelique Kerber of Germany is also through after she beat Russian Margarita Gasparyan 7-5, 6-3.

Two-time Grand Slam winner Petra Kvitova joined Kerber and Wozniacki in the next round with a 6-1 6-4 victory against Belgian Yanina Wickmayer.