His cheeks red, hair matted with sweat, Novak Djokovic appeared to be in such distress as he trudged to a changeover on a steamy US Open afternoon that someone suggested it would be a good idea to have a trash can at the ready, just in case he lost his lunch.

Djokovic sat down and removed his shirt. He guzzled water from a plastic bottle. He placed one cold towel around his neck, a second across his lap and a third between his bare upper back and the seat.

He was just over an hour into his first match at Flushing Meadows in two years, and while Djokovic eventually would get past Marton Fucsovics 6-3, 3-6, 6-4, 6-0 yesterday, it was a bit of an ordeal.

"Survival mode," he called it.

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With the temperature topping 33 degrees Celsius and the humidity approaching 50 per cent nearly everything became a struggle for every player across the grounds on Day 2 of the US Open, so much so that no fewer than five quit their matches, with three citing cramps or heat exhaustion.

About two hours into the day's schedule, the US Tennis Association decided to do something it never had at this tournament: offer men the chance to take a 10-minute break before the fourth set if a match went that far. That is similar to the existing rule for women, which allows for 10 minutes of rest before a third set when there is excessive heat.

How bad was it out there at its worst?

"Bloody hot," said two-time major semifinalist Johanna Konta, who lost 6-2, 6-2 to No 6 Caroline Garcia.

"Brutal," said 2014 US Open champion Marin Cilic, who advanced when his opponent retired in the third set.

"Really not easy," said three-time Grand Slam title winner Angelique Kerber, who defeated Margarita Gasparyan 7-6 (5), 6-3.

"Terrible. It's awful out there," said Tennys Sandgren, an American who won in straight sets and will face Djokovic in the second round. "I don't know how guys are hanging in there. I was thinking in the third set, like, 'It's getting really bad. I just don't know how long I have to play out there'. And I think everybody kind of feels similarly."

Djokovic certainly did.

"Everything is boiling — in your body, the brain, everything," said Djokovic, who's won two of his 13 Grand Slam titles in New York but sat out last year's US Open because of an injured right elbow.

He is a popular pick to hoist the trophy again, coming off a Wimbledon title in July and a victory over Roger Federer in the final of the hard-court Cincinnati Masters in August.

Federer was among those lucky enough to play a night match, facing Yoshihito Nishioka of Japan.

Djokovic was appreciative of the chance to recover a bit after the third set. He even took about a minute for a quick ice bath — as did Fucsovics, nearby.

"Naked in the ice baths, next to each other," Djokovic said. "It was quite a magnificent feeling, I must say."

In the women's draw Maria Sharapova held on to edge 39-year-old Patty Schnyder, the oldest female qualifier in Grand Slam history, 6-2, 7-6 (6) while Madison Keys eased into the second round with a 6-4, 6-4 victory over 71st-ranked Pauline Parmentier of France.