Britain’s Emma Raducanu said she needed a “tremendous effort” to beat Colombia’s Camila Osorio and reach the Washington Open quarterfinals.
Raducanu, who will start her defending U.S. Open title this month, won 7-6 (7-5) 7-6 (7-4) in two hours and 50 minutes.
“Through a game like this, it gives you a lot of confidence,” said the England number one.
Compatriot Dan Evans made progress as American Taylor Fritz retired.
In the hot and humid conditions, Raducanu and Osorio required a medical suspension.
“It was a huge effort for me to really get through,” Raducanu said.
“There are a lot of moments in both sets where you want to easily try to get that done early, but I’m really happy and proud of how I dig when it really matters.”
Radukanu will play Russia’s Lyudmila Samsonova in the quarter-finals.
Evans saved match point, but led 3-6 7-6 (8-6) 4-1 when Fritz retired, facing Japan’s Yoshio Nishioka in the quarterfinals.
Fritz said he was “embarrassed” at not being able to finish but has been wearing protective boots since last month’s Wimbledon due to a foot injury.
“I kept feeling like I was going to pass out and my vision was blurry,” he said on social media.
“The only thing that really prepares me to play in these conditions…is to play in these conditions, which I can’t do when I’m taking care of my feet.
“My feet are feeling better now and I can finally get to work on the pitch and be ready for the rest of the year. Sorry if I’ve worried anyone.”