THE SURGEON who operated on Steffi Graf's left knee on Tuesday has said that there is no guarantee that she will ever be able to resume competitive tennis.

Graf will definitely be unable to defend either her Wimbledon or US Open titles after she underwent a two-hour operation in Austria to repair cartilage and tendon damage.

The 27-year-old German star said she expected to be sidelined for four to six months following surgery.

However, her surgeon, Reinhard Weinstabl, was less certain about whether she will be able to resume her career. "That is certainly our aim," he said. "Whether that aim can be reached one cannot say now."

Winstabl said the situation was difficult because of Grafs long his tory of problems with her left knee. She was out for three months earlier this year after arthroscopic surgery on the knee.

Graf will now begin rehabilitation to establish whether she can make a full recovery from the operation, but Weinstabl added: "Whether this will succeed one cannot yet say. A first step has been made."

The surgery came less than a week after Grafs quarter-final loss in the French Open to Amanda Coetzer.

Even if she does return, Graf, who has won 21 Grand Slam titles, will have a long hill to climb to return to the top. She has dropped to number three in the WTA Tour rankings after her defeat in Paris, her lowest ranking since 1986.