Britain's Emma Raducanu said she needed a "monumental effort" to defeat 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.
In hot and humid conditions, Raducanu and Osorio required medical timeouts during the grueling encounter.
"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."
ALSO READ: Emma Raducanu Claim First Win Since US Open Triumph
Raducanu will play Russia's Liudmila Samsonova in the last eight.
On the men's circuit, Compatriot Dan Evans progressed as American Taylor Fritz retired.
MORE STORIES
Â
Â
Â
Â
