American Sloane Stephens shocked third seed Angelique Kerber to reach the Rogers Cup quarter-finals in Toronto.
Former world number 11 Stephens missed nearly a year of action because of a foot injury before returning at Wimbledon last month.
But she defeated Germany's Kerber 6-2 6-2 in 58 minutes to reach the last eight after beating number 14 seed Petra Kvitova in the previous round.
The unseeded 24-year-old will face Lucie Safarova in the next round.
New world number one Karolina Pliskova survived a scare to reach the quarter-finals as Japanese teenager Naomi Osaka retired.