Djokovic, aiming for a 14th Grand Slam title, won 6-3 3-6 6-4 6-0.

The Serb, 31, was unsteady on his feet and asked for a sick bucket to be put next to his chair in the second set.

Fucsovics then faded and Djokovic won the last 10 games, with the players taking a 10-minute heat break between the third and fourth sets.

It was the first time US Open organisers implemented an extreme heat policy in men's matches.

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

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

Wimbledon champion Djokovic, who won the men's title at Flushing Meadows in 2011 and 2015, will meet American Tennys Sandgren in the second round.

Roger Federer beat Japan's Yoshihito Nishioka 6-2 6-2 6-4 in Tuesday's night session and is a possible quarter-final opponent for Djokovic.

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Djokovic locks into 'survival mode'

Humidity has been an issue for the players in the opening two days at Flushing Meadows, where temperatures have climbed above 32C (90F) with humidity above 50%.

As well as Djokovic's issues, Fucsovics also needed the trainer as both players asked for multiple ice towels during changeovers.

The world number 41 also appeared to pick up an injury and limped away from court afterwards.

"We obviously both struggled. You could see that. Towards the end of the third we started to play a bit better, at least from my side," said Djokovic.

"Before that it was survival mode."

Djokovic and Fucsovics were the first players to enforce the heat rule for the men, breaking off two hours 22 minutes into the match, and used the time to have ice baths - in tubs next to each other.

"You're battling with a guy for two and a half hours and aren't finished and then you're naked in the ice baths," joked Djokovic.

"It was quite a magnificent feeling."

Zverev through for the loss of just five games

Alexander Zverev was a quarter-finalist at the French Open in Roland Garros this year

In other men's first-round matches, fourth seed Alexander Zverev cruised past Canada's Peter Polansky, winning 6-2 6-1 6-2 and will face Frenchman Nicolas Mahut next.

Seventh seed Marin Cilic progressed after Romanian Marius Copil retired with the Croatian leading by two sets to love.

Australian Nick Kyrgios, seeded 30th, improved as the match went out as he saw off Moldovan Radu Albot 7-5 2-6 6-4 6-2.

Veteran Frenchman Julien Benneteau, 36, knocked out Italian 22nd seed Marco Cecchinato 2-6 7-6 (7-5) 6-3 6-4.

David Goffin, the 10th seed, beat Italian Federico Gaio 6-2 6-4 7-5 (7-5) to set up a second-round tie with Robin Haase.

Dutchman Haase came from two sets down to win 4-6 4-6 6-3 6-1 6-3 against American Mackenzie McDonald in a match which lasted more than three hours.

France's Richard Gasquet beat Japan's Yuichi Sugita 6-3 6-1 6-3 while compatriot Gael Monfils defeated Argentina's Facundo Bagnis 7-6 3-6 6-0 6-0.

Japanese 21st seed and Wimbledon quarter-finalist Kei Nishikori will face Monfils in the next round after defeating German Maximilian Marterer 6-2 6-2 6-3.

But French 29th seed Adrian Mannarino was beaten 6-1 6-1 4-6 6-4 by American Frances Tiafoe on the Grandstand court.