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.
The 19-year-old qualifier, ranked 50th in the world, won a second set tie-break but called for the trainer and retired while Pliskova was leading 6-2 6-7 (4-7) 1-0.
"She played well, especially in the second set she was fighting pretty hard," said Pliskova
"I was just trying to stay in the game and I didn't know she was injured."
The Czech, 25, playing in her first event as the world's top-ranked player, won four straight games to take the first set from Osaka.
But Osaka matched top seed Pliskova shot-for-shot in the second set before dominating the tiebreak 7-4 to level the match.
During the changeover, Osaka told her coach that she was suffering from abdominal pain and was told to withdraw if the pain persisted to avoid the risk of further injury.
Pliskova will meet sixth seed and former world number one Caroline Wozniacki in the last eight.
In a battle of former Rogers Cup champions, a ruthless Wozniacki swept past 10th-seeded Pole Agnieszka Radwanska 6-3 6-1 in only 78 minutes.
Romanian second seed Simona Halep, who won the tournament last year, eased through her last-16 match with a 6-1 6-0 demolition of the 26th-ranked Barbora Strycova inside an hour.
Czech Lucie Safarova also made the last eight by beating Russia's Ekaterina Makarova 6-3 6-7 6-2, while France's Caroline Garcia beat 18-year-old American Catherine Bellis 6-4 6-2.
Later on Thursday, Ukraine's Elina Svitolina comfortably defeated American Venus Williams 6-2 6-1, setting up a meeting with Spain's Wimbledon champion Garbine Muguruza, who beat Ashleigh Barty of Australia 6-0 3-6 6-2.