Dayana Yastremska beat top seed Jelena Ostapenko to reach the Birmingham Classic quarter-finals.
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 or Elena-Gabriela Ruse of Romania next.
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.