(CNN) -- Australian Open champion Victoria Azarenka has joined the growing list of injury casualties at the WTA Rogers Cup tournament in Toronto.
The Belarus star has moved to second in the world rankings, despite losing to Samantha Stosur in the final of the Carlsbad tournament in California.
But after Sunday's straight sets defeat Azarenka pulled the plug on her appearance at the prestigious hard court tournament in Canada, citing a lower back injury.
"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."
Azarenka had only just recovered from the knee injury which forced her out of Wimbledon after the first round.
Maria Sharapova, who also sustained an injury in the second grand slam of the season, was an earlier withdrawal from the Toronto event.
It will leave both of them short of match practice on hard courts ahead of the final grand slam of the season at Flushing Meadows for the U.S. Open.
Britain's rising young star Laura Robson joined them on the sidelines before hitting a ball. The World No.32 pulled out ahead of a first round match against Belgian Yanina Wickmayer with a wrist injury.
Azarenka's withdrawal has meant she will be replaced in the draw by former World No.1 Caroline Wozniacki, with the Dane given a first round bye.
World No.1 Serena Williams is top seed as she builds up to her defense of the U.S. Open title later this month.
Meanwhile, the men's Montreal Masters also started Monday, with No.1 Novak Djokovic as defending champion and new Wimbledon champion Andy Murray as his main challenger.
First day action saw Germany's Florian Meyer beat Australia's Bernard Tomic 5-7 6-3 6-3 in a match between up and coming stars of the circuit.