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.
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.
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.
British men’s number one Cameron Norrie beat Moldova’s Radu Albot 6-3 7-6 (7-4) to reach the semi-finals of the Los Cabos Open in Mexico.
Defending champion Norrie will play Felix Auger-Aliassime in the last four. He has lost all four of his previous meetings with the Canadian.
