Two-time Australian Open winner Victoria Azarenka withdrew on Sunday from the WTA hardcourt tournament in Toronto with a lower back injury, shortly after losing a final in California.
Azarenka, who will move from third to second in the world rankings on Monday, fell 6-2, 6-3 to Australian Samantha Stosur in the final Southern California Open final in Carlsbad.
Toronto organizers, who gave Azarenka a wild card, received the news shortly afterward from the Los Angeles-based player, who met with doctors before making her announcement.
"I'm so disappointed that I can't make it to Toronto as I added it to my schedule knowing what an important event it is," Azarenka said in a statement.
"I am sorry to all my Canadian fans and I will work hard to get healthy and back on the courts as soon as possible."
In Carlsbad, Azarenka had been playing her first tournament since a knee injury forced her out of Wimbledon after one round.
The withdrawal of the three-time semi-finalist means that Caroline Wozniacki will take her spot in the draw and receive a first round bye. A lucky loser will be inserted into Wozniacki's prior place in the draw.