( 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 ca n'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 .
Carlsbad pulls out of WTA tournament in Britain . Two -time Canadian Open winner has a back injury . She lost in the final of Carlsbad tournament to Samantha Stosur . Montreal Masters starts Monday with Victoria Azarenka cefending title .