Britain’s Emma Raducanu said she needed a “monumental effort” to beat Colombia’s Camila Osorio and reach the Washington Open quarter-finals.
Raducanu, who will start the defence of her US Open title this month, won in two hours 50 minutes.
“For me it was a pretty monumental effort to really get through that,” said Raducanu.
Compatriot Dan Evans progressed as American Taylor Fritz retired.
In hot and humid conditions, Raducanu and Osorio required medical timeouts.
Raducanu will play Russia’s Liudmila Samsonova in the last eight.