World number one Aryna Sabalenka and Jessica Pegula were both upset in fourth-round matches on Saturday at the Toronto Masters.
Four-time Grand Slam winner Sabalenka suffered a blow to her preparations for a tilt at a third consecutive US Open title, beaten 7-6 (7/3), 4-6, 6-4 by Russian 16th seed Ekaterina Alexandrova.
"I wasn't trying to think about the score or anything like beating number one," said Alexandrova after ousting the Belarusian in 2hr 29min.
"I was just trying to do my best.
"I just tried to play every single point as if it was the last one because with her, you don't get a lot of chances during the match," AFP quoted Alexandrova as saying.
"I had a lot today, but I couldn't realize some of them. I'm super happy that I could win because after the second set, I thought (winning) was already past me."
Alexandrova moves on to a quarter-final against Ukraine's Elina Svitolina, who dispatched American Amanda Anisimova 6-2, 6-4 in 78 minutes.
Russian 15th seed Shnaider upset world number three Pegula 6-3, 6-3 to book a last-eight date with Poland's Iga Swiatek.
The 22-year-old left-hander took only 73 minutes to defeat Pegula.
"This week I've been very comfortable on both defending and just firing some winners," Shnaider said.
"It was working well. I was choosing a bigger target today but sometimes ripping the ball."
Sabalenka dropped the first set on a tiebreak as her Russian opponent won five of the last six points.
"I was trying to play super aggressive, the way I didn't play for a long time. It worked," said Alexandrova.
"I had the feeling I have some kind of power and I can create something and it helped me mentally."
Sabalenka levelled the match but in the decider faced match points on her serve in the 10th game.
She saved the first on a service winner and the second with an ace.
But, after sending a backhand beyond the baseline, the Belarusian made her sixth and final double fault to lose the match.
"I didn't have any expectations before the match," Alexandrova said. "I knew it was going to be super difficult when I walked on the court.
"I was trying to risk as much as possible because it felt like it was the only way to create some chances. It caused many more mistakes but at the same time I created many more opportunities."
Former world number one Swiatek, the seventh seed, rallied to defeat Ukraine's 10th-seeded Marta Kostyuk 3-6, 6-1, 6-2.
Swiatek, a four-time French Open champion, saved nine break points in the last two sets without dropping serve.
"In the second I came out... playing a bit faster and more heavy and pushing Marta a little bit more," Swiatek said.
"Game by game I felt like it was a good plan and a good choice and I went for it."
