WIMBLEDON champion Steffi Graf is out of this year's championships after undergoing a knee operation in Vienna yesterday that will sideline her at least four months.

The 27-year-old German, who slipped to third in the world rankings after losing in the French Open quarter-finals last week, was forced to have the operation as her kneecap was pressing against and irritating a tendon.

Graf, who was out of action for over three months earlier this season with another knee injury, went straight to a rehabilitation clinic following the operation.

Graf also feels that is unlikely she will play any further grand slam tournaments this year.

"When exactly I will be able to compete again is not clear, she said. "The doctors told me that with extensive physiotherapy, returning in four to six months is realistic. It seems unlikely that I will be able to defend my US Open title as well (as Wimbledon)."