Defending champions Djokovic, Gauff advance at steamy US Open

Defending champion Novak Djokovic reached the third round of the US Open on Wednesday as Laslo Djere retired in the third set, but said he'll have to do better to win a fifth title on the hard courts of New York.

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Djokovic was up 6-4, 6-4, 2-0 when Djere pulled the plug on a gritty encounter that saw both men summon the physio to Arthur Ashe Stadium.

"It's not what we want to see," Djokovic said. "He's such a good player in these conditions and the second set should have been his, he was 4-2 up.

"I don't know if (my) winning the second set probably put more burden on him."

Djokovic notched his 90th US Open match victory, becoming the first man to win 90 at all four Grand Slams.

But it was a battle until the minute Djere called it quits.

Djokovic grabbed the first break of the match to take the first set 6-4 after a tense hour then sought treatment for trouble on his right side.

Djere, the only player to take a set off Djokovic at last year's US Open, gained the first break of the second set and with a 4-2 lead had two more break points that he couldn't convert.

But Djokovic won the next six games, Djere receiving treatment on his abdomen before Djokovic closed out the second.

"Overall it was a big fight – over two hours for two sets," Djokovic said.

"I served awful. So playing without the serve, you have to grind, you have to run. I had to rely on my baseline game."

The Serbian superstar, coming off an emotional Paris Olympics triumph, next faces Australian Alexei Popyrin, who he beat at the Australian Open and Wimbledon this year.

Women's defending champion Coco Gauff overcame her own struggles on serve to beat 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 per cent of her first serves in play and had nine double faults, but the 99th-ranked Maria couldn't capitalise 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 and Gauff missed the worst conditions of a steamy day that saw tournament organisers invoke the extreme weather rule allowing mid-match breaks.

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.

"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.

Fourth-seeded Alexander Zverev of Germany advanced with a 6-4, 7-6 (7/5), 6-1 victory over France's Alexandre Muller.

"I am happy to be done in three sets, to have some rest as it is very hot and tough conditions," said Zverev, who said he felt fine but "was at some point very wet."

Zverev next faces Argentina's Tomas Etcheverry, whose efforts to stay hydrated backfired during his 6-3, 4-6, 6-4, 1-6, 6-3 victory over compatriot Francisco Cerundolo.

DANGEROUS CONDITIONS

"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," said Etcheverry, who fired 23 aces to finally seal the win after more than four hours.

"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 quarterfinalist, 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.

Rublev also called medical staff after drinking so much water he felt like he had "a baby" in his stomach.

Elsewhere, seventh-seeded Paris Olympics gold medallist Zheng Qinwen had to go the distance, rallying from a set down for the second straight match to beat Russian Erika Andreeva 6-7 (3/7), 6-1, 6-2.

But 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.