Jack Sock has exited the US Open after collapsing on court against Belgian Ruben Bemelmans.
The 28th seed is believed to have collapsed due to heat exhaustion, and is the 11th player to retire injured this week.
Sock had taken a two-set lead before slowing his play down in the third set. Three games into the fourth the American went down and appeared to be fading in and out of consciousness.
Temperatures at Flushing Meadows have, at times, risen to 35 degrees Celsius.