Reigning Wimbledon champion Petra Kvitova and French Open winner Li Na of China both fell to straight sets defeats in the pre-US Open WTA hardcourt tournament, Rogers Cup, on Thursday.
Czech seventh seed Kvitova was beaten 6-1, 6-2 by Germany's Andrea Petkovic, the 11th seed, while Li, seeded sixth, went out 6-2, 6-4 to 10th seeded Samantha Stosur of Australia.
Russian fifth seeded Maria Sharapova also crashed out in straight sets, losing 6-3, 7-5 to Kazakh qualifier Galina Voskoboeva.
Having recovered from a thigh muscle injury, the 21-year-old Kvitova was starting her overseas campaign in Toronto which had expected to be topped by the US Open at the end of August.
Li, 29, made sporting history at the French Open in June when she became the first player from China to win a Grand Slam singles title.
Li, who was playing her first match of the week after a bye and a walkover victory when Chinese compatriot Peng Shuai withdrew with an injury, said she felt "like a junior on the court."
"It's always tough after a break to come back for the first match, because I had six or seven weeks that I didn't play a tournament," Li said. "At the beginning of the match I didn't even know what I should do on the court, not like during the clay-court season."
Li bowed out when she hit a forehand into the net, ending the 89-minute match in windy conditions at the Rexall Centre venue.
"Today was so, so windy, I couldn't use my serve a lot in the first serves," Li said. "If you watched the match, everyone can see, she has a huge, big serve. It's tough for me to return."
Li, Sharapova and Kvitova joined top-ranked Caroline Wozniacki on the sidelines. Wozniacki was upset by No. 22 Roberta Vinci on Wednesday.