World number two Simona Halep was forced to retire from her Washington Open quarter-final against Ekaterina Makarova with a heat-related illness.
After complaining that severe heat left her feeling "a bit dead" on Wednesday, Romanian Halep was trailing 2-6 6-3 1-0 when she withdrew in the third set.
The on-court temperature reached a punishing 47C on Wednesday.
In the men's event, Canadian Milos Raonic lost 7-5 6-4 to USA's Jack Sock in their quarter-final.
World number 10 Raonic has now gone 30 tournaments since lifting his last trophy in Brisbane in 2016 - a drought of 19 months.