World number one Caroline Wozniacki suffered a shock defeat to Italian Roberta Vinci in her opening match at the Rogers Cup in Toronto.
Returning to defend her title, Wozniacki received a first-round bye but lost 6-4 7-5 to Vinci, wasting the chance to level the match when she had three set points during the second set.
Wozniacki, who had not played since losing a second-round match in Bastad last month, told wtatour.com: "I would have loved to win my first match back, especially after leading 5-1 in the second set. It's not fun to lose this one."
Playing her first match since winning her breakthrough Wimbledon title, Petra Kvitova had no such problems as she beat Spaniard Anabel Medina Garrigues 7-6 (7/3) 6-3 in one hour and 37 minutes.
Meanwhile Serena Williams continued the form which saw her sweep to the title in Stanford at the end of last month as she claimed a 6-1 7-6 (9/7) victory over Germany's Julia Goerges.
Wozniacki apart, there were no seeded casualties, with eighth seed Francesca Schiavone, 10th seed Samantha Stosur and 13th seed Agnieszka Radwanska all advancing in early action.
Kazakh qualifier Galina Voskoboeva, who beat Marion Bartoli in the first round, continued her fine run with a 2-6 6-3 6-2 win over Flavia Pennetta, and there were also wins for Lucie Safarova and Maria Jose Martinez Sanchez.
Russian fifth seed Maria Sharapova, beaten by Kvitova in the Wimbledon final, was a 6-1 7-5 winner against Serbian Bojana Jovanovski.
Sharapova's fellow Russian, third seed Vera Zvonareva, beat compatriot Nadia Petrova 7-5 6-4.
Serbian Ana Ivanovic continued her bright start to the tournament with a 6-3 6-2 success over Czech Iveta Benesova, and German 11th seed Andrea Petkovic won 6-4 7-5 against Hungary's Greta Arn.
Returning to defend her title, Wozniacki received a first-round bye but lost 6-4 7-5 to Vinci, wasting the chance to level the match when she had three set points during the second set.
Wozniacki, who had not played since losing a second-round match in Bastad last month, told wtatour.com: "I would have loved to win my first match back, especially after leading 5-1 in the second set. It's not fun to lose this one."
Playing her first match since winning her breakthrough Wimbledon title, Petra Kvitova had no such problems as she beat Spaniard Anabel Medina Garrigues 7-6 (7/3) 6-3 in one hour and 37 minutes.
Meanwhile Serena Williams continued the form which saw her sweep to the title in Stanford at the end of last month as she claimed a 6-1 7-6 (9/7) victory over Germany's Julia Goerges.
Wozniacki apart, there were no seeded casualties, with eighth seed Francesca Schiavone, 10th seed Samantha Stosur and 13th seed Agnieszka Radwanska all advancing in early action.
Kazakh qualifier Galina Voskoboeva, who beat Marion Bartoli in the first round, continued her fine run with a 2-6 6-3 6-2 win over Flavia Pennetta, and there were also wins for Lucie Safarova and Maria Jose Martinez Sanchez.
Russian fifth seed Maria Sharapova, beaten by Kvitova in the Wimbledon final, was a 6-1 7-5 winner against Serbian Bojana Jovanovski.
Sharapova's fellow Russian, third seed Vera Zvonareva, beat compatriot Nadia Petrova 7-5 6-4.
Serbian Ana Ivanovic continued her bright start to the tournament with a 6-3 6-2 success over Czech Iveta Benesova, and German 11th seed Andrea Petkovic won 6-4 7-5 against Hungary's Greta Arn.