France's Arthur Rinderknech sparked worrying scenes at the Cincinnati Open on Monday after collapsing on the court mid-match.
The 30-year-old was later forced to retire from his match against Canada's Felix Auger-Aliassime having struggled with the sweltering heat.
Several tennis players have been forced to combat near-unplayable conditions as temperatures reached 32 degrees Celsius on Monday.
Rinderknech was contesting a round-of-32 match against Auger-Aliassime when the scorching heat proved too much.
The Frenchman, who dropped the first set of his third-round encounter, battled back to seal an early break-up in set two.
However, after instantly surrendering his service to make it 2-2, Rinderknech then collapsed as he moved to grab a towel.
The umpire could be heard saying: "Arthur