TORONTO (AFP) - Caroline Wozniacki's bid for a second-straight WTA Canada crown ended abruptly on Wednesday as the world number one was stunned in her opening match by unseeded Italian Roberta Vinci. Vinci defeated the top-seeded Dane 6-4, 7-5 to reach the third round. Wozniacki, playing her first tournament since Bastad in the first week of July, had enjoyed a first-round bye. After dropping the first set, Wozniacki mounted a challenge, taking a 5-1 lead in the second. She was particularly disappointed that she couldn't at least win the second set to give herself a chance in a third. Wozniacki and Clijsters led a strong field that included all of the WTA's top 20 players here this week. But one of the players most in the spotlight isn't even in that group, as Serena Williams arrived unseeded in the wake of her 11-month injury layoff that ended in June. The former world number one defeated Germany's Julia Goerges 6-1, 7-6 (9/7) to set up a meeting with Zheng. Williams also said the gusty wind made things difficult. "If you hit it too hard the ball is going to totally fly the harder you hit." Third-seeded Vera Zvonareva, fourth-seeded Victoria Azarenka and fifth-seeded Maria Sharapova all advanced, as did French Open champion Li Na and Wimbledon champion Petra Kvitova. Zvonareva defeated fellow Russian Nadia Petrova 7-5, 6-4, while Sharapova survived some erratic serving in the second set to defeat Serbian qualifier Bojana Jovanovski 6-1, 7-5. Azarenka, of Belarus, ended the hopes of Canadian Stephanie Dubois 6-0, 6-0. China's Li advanced on a walkover when compatriot Peng Shuai withdrew prior to their match with a left hip injury suffered in San Diego last week. "It is sad for her," said Li, the sixth seed. "Always tough when you're playing same-country player. We train together, we know much (about each other). Always tough."