World number 19 Ekaterina Alexandrova collapsed on court and had to retire from her first-round match at the Memphis Classic due to high temperatures.
Russian Alexandrova, who was the number one seed, was trailing 7-6 (7-2) 4-6 5-4 against 16-year-old compatriot Kristina Liutova in a match where temperatures reached 34C.
The match was initially paused as Alexandrova tried to continue, before she received a time violation as she took too long to serve amid her struggles.
But in the middle of the next point, Alexandrova fell to the ground and needed immediate medical attention.
The 31-year-old was well enough to stand but had to forfeit the match, which had lasted more than three hours.
It gave Liutova victory in her debut match on the WTA tour, but the teenager was left in tears on court amid the chaotic situation.
Elsewhere in women’s tennis, Venus Williams lost in the first round of the Mubadala DC Open in Washington DC on Tuesday.
The 46-year-old, whose last singles win came at this event last year, was beaten 6-3 6-3 by Austria’s Anastasia Potapova.
Williams won 78.4% of her first serve points but just 24.1% from her second serve, and 20% of her returns.