- Victoria Azarenka is struggling for full fitness after heavy schedule this season
- Australian Open champion will also miss next week's Cincinnati Masters
- Belarus star is hoping to make it past fourth round at Flushing Meadows for first time
- Second-ranked Maria Sharapova withdraws from second consecutive event due to illness
World No. 1 Victoria Azarenka is facing a race against time to be fit for the U.S. Open after succumbing to the frenetic schedule of women's tennis.
The Belorussian was forced to withdraw from the Montreal Cup with an injury to her left knee midway through the first set of her opening match against Tamira Paszek on Thursday.
With just 17 days until the start of the competition at Flushing Meadows, the Australian Open champion's hopes of adding to her sole grand slam triumph look slim.
The 23-year-old reached the semifinals of Wimbledon last month, before going on to compete at the London 2012 Olympics where she won bronze in the singles and gold in the mixed doubles on Sunday.
The workload has taken its toll on Azarenka, who will now also miss next week's Cincinnati Masters event.
"The last week took so much energy out of me that I had absolutely no time to recover, and my body is feeling it," she told reporters.
"It was a little bit too quick of a transition for me to come and start the training, and I felt my knee just wasn't in good shape.
"It's unfortunate because I was really looking forward to playing here. I have really enjoyed being here. I tried yesterday and today it just got worse. I can't see the point of continuing to play if I can't give even 50% of my game."
Azarenka, who has never made it past the fourth round at the U.S. Open, said she needs rest and recuperation if she is to line up in New York later this month.
"I hope it's a short-term thing. I have to just stop and take a little bit of time off because I had absolutely no rest for a long period of time. I just need to settle it down and go from there. Hopefully there's nothing serious about it," she said.
"If I cannot play here, there is no chance for me to play anywhere else. My first goal is to get healthy and make sure I'm 100% ready to play the U.S. Open."
The hectic schedule claimed another victim when world No. 2 Maria Sharapova also pulled out of the WTA Tour event in Cincinnati.
The French Open champion, who was defeated in the Olympic final by Serena Williams, missed Montreal due to a stomach virus.
The Russian hopes to recover in time for the U.S. Open, which she won in 2006.