Defending champion Victoria Azarenka has withdrawn from the Western & Southern Open because of a right knee injury.

The WTA says Azarenka withdrew after practicing on Tuesday. Azarenka was replaced in the draw by Mona Barthel.

Azarenka injured her knee in her first-round win at Wimbledon and played in two tournaments after the grass-court major.

Azarenka's withdrawal is the latest setback in a season full of injuries. She has been sidelined for much of the year with an ailing left foot. She aggravated an injury to her right knee at Montreal, where she lost in the quarterfinals, and hoped a few days of rest would take care of it.

She decided to withdraw after practicing Tuesday morning.

"I started to feel a little bit better and did everything I could, but it's just not enough time for me to feel good to play a full match," she said. "I'm going to get more treatment and get more work done and should be good for the U.S. Open."

The U.S. Open starts Aug. 25.