Tunisia's Ons Jabeur rallied from a set down to knock second-seeded defending champion Bianca Andreescu out of the WTA hardcourt tournament in Montreal 6-7 (5/7), 6-4, 6-1 on Thursday.
The third-round upset was twice interrupted by rain as the Birmingham champion Jabeur - the first Arab woman to win a WTA title - completed the second defeat of the day for Canada.
Earlier, top seed Aryna Sabalenka dismissed 220th-ranked local Rebecca Marino 6-1, 6-3 in an hour.
Jabeur won her 36th match of the season, second-best on the Tour behind Sabalenka, with 37.
The 13th seeded Jabeur ripped through the third set, advancing on her second match point. She broke former US Open champion Andreescu six times.
"Great match," Jabeur said. "It was a tough one. I know coming to play Bianca here in Montreal, in Canada, was going to be difficult."
Jabeur said she sensed fatigue in her rival, who received a medical timeout late in the second set after coming down awkwardly on her left foot.
"I think she got tired a little bit," Jabeur said. "I can see that from the second set. It's completely normal. Honestly, I didn't focus a lot on what she was having.
"I was more focusing on myself, of what I should do. I really focused on returning better, serving better. It helped me a lot in the third set."
Jabeur next faces American Jessica Pegula who needed six match points to get past red-hot compatriot Danielle Collins 6-4, 3-6, 7-5.
Collins arrived in Montreal having won back to back titles in Palermo and San Jose, but she finally couldn't hold off Pegula in a match that lasted two hours and 28 minutes and wrapped up at 1:30 in the morning.
Sabalenka will play for a semi-final spot against double Grand Slam champion Victoria Azarenka -- a 6-4, 3-6, 7-6 (7/2) winner over Maria Sakkari -- in an all Belarussian clash.
Azarenka will be playing the Canadian quarters for the first time in seven years.
"It was a very good battle with a lot of rallies," Azarenka said. "Maria played very well.
"I was able to take advantage in the important moments of the third set. After taking a break (since Wimbledon) those matches where I have to find the winning solution are very important."
Sabalenka has won two of three prior meetings with Azarenka, a compatriot nine years her senior who counts a pair of Australian Open titles in her trophy collection.
Sabalenka halted Marino's progress as the Canadian continues to resurrect a career she interrupted for five years to work out mental health concerns.
Sabalenka broke Marino four times while saving all five break points she faced.
Sabalenka, who has not lost a match to a player outside the Top 200 since June 2019, will be playing her eighth quarter-final of the season.
Fourth seed Karolina Pliskova duplicated her Canadian career-best by reaching the quarters with a 6-1, 7-6 (10/8) victory over Amanda Anisimova.
Pliskova next faces Spain's Sara Sorribes, who ousted Czech Katerina Siniakova 6-7 (4/7), 6-0, 6-3.
Seventh-seeded Czech Petra Kvitova was eliminated by Italy's Camilla Giorgi 6-4, 6-4.
Britain's Johanna Konta, who upset third seed Elina Svitolina in the third round, withdrew with a knee injury, handing a walkover to Coco Gauff -- who will take on Giorgi for a place in the semis.
The 17-year-old American has played only one full match so far this week after Russian Anastasia Potapova quit their third-rounder with an ankle problem.