Dayana Yastremska beat top seed Jelena Ostapenko to reach the Birmingham Classic quarter-finals.
The Ukrainian beat Latvia's Ostapenko 3-6 7-5 7-5 in a big-hitting and, at times, scrappy contest.
Yastremska hit nine double faults to 14 aces but twice fought back from a break down in the final set to advance.
"We are very good friends and it has been hard to play against her in a personal way as well as a tennis [way]," Yastremska, 22, said.
The world number 79 will face China's Zhang Shuai, who beat Elena-Gabriela Ruse 6-2 2-6 7-5.
It is the first time Yastremska has reached a grass-court quarter-final on the WTA Tour.
She received vocal support from the Birmingham crowd and made a heart symbol with her hands to thank them after her victory.
"We are both crazy players, so you don't know what to expect from both of us," she said.
"I felt amazing support today and it felt super nice, especially the third set at the end. It's been a hard match and I just tried my best."
Earlier, Italian third seed Camila Giorgi secured her quarter-final place with a 3-6 7-5 6-2 win over American Lauren Davis.
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