New York - America's Jack Sock fainted in 33-degree heat and was helped off court at the US Open on Thursday.
The 22-year-old 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 becomes the 11th man to retire from a match at this year's tournament, which has been played in 30-plus heat every day.
"It's a shame as he was playing really well in the first two sets," said Bemelmans.
"It has happened to me in Davis Cup -- it's not a nice feeling."