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 7-6 (7-5) 7-6 (7-4) in two hours 50 minutes.
"It just gives you a lot of confidence coming through a match like that," said the British number one.
Compatriot Dan Evans progressed as American Taylor Fritz retired.
In hot and humid conditions, Raducanu and Osorio required medical timeouts.
"For me it was a pretty monumental effort to really get through that," said Raducanu.