Qualifier Galina Voskoboeva upset France's Marion Bartoli 6-3, 6-3 to reach the second round of the $2.05 million WTA hardcourt tournament on Monday.
Kazakhstan's Voskoboeva, currently ranked 135th in the world after reaching a career high of 64th in February of 2009, arrived in Toronto after reaching the first semifinal of her career at Baku, Azerbaijan, in July.
Bartoli, ranked ninth in the world and seeded ninth, had trouble with her serve and double-faulted four times. But she said her serve was just one of her problems.
"It was a day when nothing was working," Bartoli said. "I tried my hardest but I committed so many mistakes it was hard to win. I was not feeling the ball and my legs were not moving.
"I was late on everything and committing a lot mistakes, especially on my backhand."
Romanian qualifier Simona Halep pulled off another upset, rallying to defeat 12th-seeded Russian Svetlana Kuznetsova 4-6, 6-4, 6-3.
Kuznetsova, ranked 15th in the world, had 50 unforced errors.
Tenth-seeded Samantha Stosur of Australia, who dropped out of the top 10 on Monday for the first time since April of last year, advanced with a 4-6, 6-2, 6-3 victory over Ayumi Morita of Japan.
Stosur will meet the winner of Tuesday's match between Aleksandra Wozniak of Canada and Shahar Peer of Israel.
The top eight seeds at this US Open tuneup have first-round byes.
The talent-laden field is led by world number one Caroline Wozniacki, trying to get back on track after early exits at Roland Garros and Wimbledon.
Last year Wozniacki went all the way to the title here and after a first-round bye, she will begin her defense against either Yanina Wickmayer or Roberta Vinci.
World No.2 Kim Clijsters is the second seed but has played just two tournaments in the past four months as she battled a string of injuries to her shoulder, wrist and ankle.
In the second round on Tuesday Clijsters will face China's Zheng Jie, a 6-2, 6-1 winner on Monday over fellow qualifier Alberta Brianti of Italy.
Russian Nadia Petrova also advanced, defeating Australian Jarmila Groth 6-1, 6-2 to set up a second-round clash with third-seeded compatriot Vera Zvonareva.
Victoria Azarenka of Belarus, Russian Maria Sharapova, China's Li Na, Czech Petra Kvitova and Italy's Francesca Schiavone round out the top eight seeds.
The field also includes 13-time Grand Slam champion Serena Williams, who is unseeded after returning from an absence of nearly 11 months because of injury and illness.
She cemented her return by lifting the title in Stanford a fortnight ago.
If Williams gets past Alona Bondarenko in the first round on Tuesday night she'll face Germany's Julia Goerges, a 6-1, 6-3 winner over Serbian 15th seed Jelena Jankovic.