American Jack Sock fainted in 33-degree heat and was carried off court at the US Open on Thursday as the number of men quitting hit a Grand Slam record 12.
The 22-year-old Sock was leading Belgium's Ruben Bemelmans 6-4, 6-4, 3-6, 1-2 when he started cramping and was forced to retire.
Sock, the 28th seed, then collapsed on the court, where he was treated with ice packs and cold towels by US Open medical staff before being helped off to the shade of the Grandstand Court.
He became the 11th man to retire from a match at this year's tournament, which has been played in 30-plus heat every day.
Just hours later, Denis Istomin of Uzbekistan became the 12th when he also called it quits on Court Seven against Austrian 20th seed Dominic Thiem, who was 6-4, 6-4, 1-0 up at the time.
Two women also retired in the first round, taking the total for this year's US Open to 14.
Temperatures have been over 30 degrees (86 degrees Fahrenheit) all week with humidity hitting 40% on Thursday at a steamy Flushing Meadows.
The carnage led again to suggestions that the men, who play the best of five sets, should be accorded the same protection as women players.
The WTA allows a 10-minute break between the second and third sets of women's matches when the mercury bursts through 30.1 degrees.
"I honestly don't know what I want to see. I'm normally OK with heat and humidity," said French Open champion Stan Wawrinka.
"I don't have problem physically. Never retired because of that. Never had really big cramping or big problem because of that.
"For me, it's fine. I'm still OK with no rule."
Sock later said he had recovered and was looking ahead to playing in the Davis Cup play-off tie against Uzbekistan in Tashkent later this month.
"Playing in the US Open is the biggest and most important moment of the season for me, so having to retire from my match today is extremely disappointing," he said in a statement.
"I want to thank everyone for their support and can't wait to be back next year. I feel better already and look forward to playing Davis Cup."
Britain's world number 97 Johanna Konta shocked Wimbledon runner-up Garbine Muguruza in the longest women's match in US Open history, which was played at the hottest time of the day out on Court 17.
"I thought it would be really embarrassing if I just toppled over here," said Konta, who won 7-6 (7/4), 6-7 (4/7), 6-2 in 3 hours 23 minutes.
It beat by seven minutes the previous record set in 2011 when Samantha Stosur defeated Nadia Petrova.
"It's a horrible situation to be in," added Konta.
"It was definitely hot out there. I'm already a sweater so I was really perspiring. I dealt with it the best I could."
Bemmelmans said he sympathised with Sock.
"It's a shame as he was playing really well in the first two sets," said the Belgian.
"It has happened to me in Davis Cup -- it's not a nice feeling."
The sight of Sock being carried off also sparked calls for a new look at the scheduling especially for players involved in other events.
"@usopen if we want men's singles players @JackSock to keep playing doubles then scheduling needs to improve or even fewer will play both," tweeted former US player turned television pundit Pam Shriver.