Novak Djokovic engaged survival mode as he battled past Marton Fucsovics in brutally hot and humid conditions at the US Open.
Four players retired mid-match as conditions took another step up in intensity on Tuesday and for a while it looked as if Djokovic might become the fifth before he recovered to win 6-3 3-6 6-4 6-0.
He called the doctor during the second set and trailed by a break in the third but took advantage of the tournament's new extreme heat rule for the men, allowing a 10-minute break between the third and fourth sets, and returned to the court refreshed.
Djokovic, who has found similar conditions difficult in the past, said: "We obviously both struggled. It was survival mode. Credit to Marton, he's a great fighter.
"I was actually praying that the next moment I get to feel better. I definitely wasn't feeling great for most of the three sets. But you have these kind of days.
"I'm not the only one, a lot of the players struggled. I'll take the win."
Regarding the heat rule, Djokovic said: "I want to thank the US Open for allowing us to have a 10-minute break.
Marton and I were in ice baths next to each other. We were naked in the ice baths and it was a quite wonderful feeling.
Djokovic arrived in New York as many people's favourite for the trophy after beating Roger Federer to become the first man to win all the nine different Masters titles in Cincinnati last weekend.
It was smooth sailing initially against Fucsovics as Djokovic won the first set but he soon began to look fatigued and very uncomfortable in the energy-sapping conditions.
Perhaps aware he needed to finish the match quickly, the sixth seed destroyed a racket after failing to break the Fucsovics serve for 3-2 in the second set, and the Hungarian won the next two games as well.
Djokovic called for the doctor and asked for a bin to be put next to him because he was feeling so nauseous. The 31-year-old was in real trouble when he went a break down early in the third set but he dug in to limit his losses and was urged on by his support camp, including wife Jelena.
A fillip came just in time as Fucsovics suddenly began to wilt, also calling for the doctor, and Djokovic won four games in a row to clinch the third set before both players gratefully retreated to the cool of the locker room. Djokovic looked a different man on the resumption and did not drop another game as he booked a second-round date with American Tennys Sandgren.