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.
“There were many moments in both sets where you want to go for the easy option of trying to finish the point early, but I’m just really pleased and proud of how I dug in when it really mattered.”
Raducanu will play Russia’s Liudmila Samsonova in the last eight.
Evans, who saved match point but was leading 3-6 7-6 (8-6) 4-1 when Fritz withdrew, faces Yoshihito Nishioka of Japan in the quarter-finals.
Fritz said he felt “embarrassed” at not being able to finish the match, but that he had been wearing a protective boot since Wimbledon last month because of a foot injury.
Evans, Raducanu seal Q/F spots