Novak Djokovic will aim for a fast start against fellow Serbian Laslo Djere on Wednesday as he steps up his bid for a record 25th Grand Slam singles title at the US Open.

Djere, ranked 109th in the world, was the only player to take a set off Djokovic at last year's US Open, taking the first two of their third-round clash before Djokovic turned the tide on the way to a 24th major crown, which tied him with Margaret Court for most all-time.

Djokovic, coming off an emotional triumph at the Paris Olympics, anchored a night session on Arthur Ashe Stadium that opened with defending women's champion Coco Gauff beating 37-year-old Tatjana Maria 6-4, 6-0.

It was an uneven performance from 20-year-old Gauff, who has endured an erratic season since capturing her maiden major in New York.

She put just 44 percent of her first serves in play and had nine double faults, but the 99th-ranked Maria couldn't capitalize and Gauff won the last seven games to seal the win.

"I think I played well overall," Gauff said. "I think if I could have served better that first set would have been a lot easier."

Djokovic, 37, had quipped that while he enjoys the energy of night sessions he could do with an earlier bedtime.

But he could be grateful to miss the worst heat of a steamy day that saw tournament organizers invoke the extreme weather rule and allows mid-match breaks to distressed players in long matches.

Australian Open champion Aryna Sabalenka, runner-up to Gauff last year, didn't have to take advantage, polishing off Italian Lucia Bronzetti 6-3, 6-1 in an hour.

The world number two from Belarus, aiming to become the first woman since Angelique Kerber in 2016 to win both hard court Grand Slam titles in the same year, said she was determined to make it a quick.

"I told myself you have to stay focused from the first point to the last point and make sure you're not going to stay here a crazy number of hours," Sabalenka said.

Seventh-seeded Paris Olympics gold medallist Zheng Qinwen had to go the distance, rallying from a set down for the second straight match to reach the third round with a 6-7 (3-7), 6-1, 6-2 victory over Russian Erika Andreeva.

Eighth-seeded Wimbledon champion Barbora Krejcikova became the biggest upset victim of the tournament so far, bounced 6-4, 7-5 by Romanian qualifier Elena-Gabriela Ruse.

- Amazing tennis -

In men's action, fourth-seeded Alexander Zverev of Germany advanced on cue with a 6-4, 7-6 (7/5), 6-1 victory over France's Alexandre Muller.

"I thought he played some amazing tennis. Especially at the end of the second set, I thought he was the better player than me," Zverev said. "I am happy to be done in three sets, to have some rest as it is very hot and tough conditions."

Zverev next faces Argentina's Tomas Etcheverry, who appeared to be feeling the heat when he vomited on court during his 6-3, 4-6, 6-4, 1-6, 6-3 victory over compatriot Francisco Cerundolo.

Etcheverry fired 23 aces to finally seal the win after more than four hours.

"The conditions were very difficult, honestly," Etcheverry said. "We were expecting them, but the temperature was 38 degrees and to that we have to add another four degrees inside the court.

"You had to hydrate well, try to take as many salts and hydrates as possible and I overdid it, that's why I ended up vomiting.

"It's dangerous not only for the players, but also for the public," he added.

Sixth-seeded Russian Andrey Rublev and Czech Jiri Lehecka both battled through five sets to line up a third-round meeting.

Rublev, a four-time US Open quarter-finalist, beat France's Arthur Rinderknech 4-6, 5-7, 6-1, 6-2, 6-2 while Lehecka out-lasted American Mitchell Krueger 6-7 (5/7), 0-6, 6-4, 6-4, 7-5.

Ukrainian Marta Kostyuk had little energy to celebrate her 7-6 (12/10), 6-1 victory over Britain's Harriet Dart, making her way to her courtside chair and re-applying ice packs to her head and neck.