Photo: David Ramos/Getty

Defending U.S. Open champion Stan Wawrinka announced today he is undergoing a “medical intervention” on his knee and will miss the rest of the 2017 season.

“This was the only solution to make sure I will be able to compete at the top level for many more years,” the current world No. 4 said in a statement.

Wawrinka had a nagging knee injury before Wimbledon and was upset in the opening round of the tournament by Daniil Medvedev. Earlier this week, Wawrinka pulled out of two U.S. Open tuneup tournaments—the Rogers Cup in Montreal followed by the Cincinnati Masters.

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With Novak Djokovic out for the season with an elbow injury, and Andy Murray nursing a hip injury that forced him to withdraw from next week’s Rogers Cup, three of the sport’s big five players could be out for the U.S. Open. That could leave the door wide open for one of the young guns or, tennis gods willing, a rematch of the Australian Open final between Roger Federer and Rafael Nadal.