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.
But Vinci, ranked 22 in the world, battled back to finish off the victory.
"This is the best victory in my life," said Vinci, 28. "I was 5-1 down but I knew that I can win the set because it was a lot of wind, so some mistakes.
"I tried to stay focused and play aggressive."
Wozniacki, winner of a WTA-leading five titles this season, had trouble with her serve in the windy weather. The conditions contributed to her seven double faults.
"It was very difficult at first with the wind," Wozniacki said. "Especially when you threw it up. It was going everywhere, so definitely that didn't make it easy. It was blowing in different directions, but it's the same for both players."
She was particularly disappointed that she couldn't at least win the second set to give herself a chance in a third.
"I would have liked to have won that one and especially after leading 5-1 in the second," Wozniacki said.
Her early exit leaves the tournament without its top two seeds after second-seeded Kim Clijsters retired from her match against China's Zheng Jie with a stomach muscle injury that casts doubts on the Belgian's upcoming US Open title defence.
Eighth-seeded Francesca Schiavone of Italy also reached the third round with a 6-1, 6-3 victory over Ekaterina Makarova.