WASHINGTON (AFP) — Reigning US Open champion Emma Raducanu of Britain advanced to the quarterfinals of the ATP and WTA Washington Open, refusing to wilt Thursday after a marathon match in sweltering heat.

The English 19-year-old outlasted Colombia's Camila Osorio 7-6 (7/5), 7-6 (7/4) after two hours and 50 minutes for a gritty victory in the WTA's longest two-set main draw match of the year.

"It has got to be one of the most physical matches I've ever played," Raducanu said.

"It was a pretty monumental effort to really get through that."

"I'm just really pleased and proud of how I dug in when it really mattered. It just gives you a lot of confidence coming through a match like that."

"Physically, I'm pretty pleased with how I held up in that match. Looking forward to going again."

Raducanu advanced to a Friday quarterfinal against Liudmila Samsonova, who rallied past Australian Ajla Tomljanovic 4-6, 6-3, 6-2 at the US Open tune-up.