Stan Wawrinka out for season after knee surgery

World No. 4 Stan Wawrinka of Switzerland will not defend his U.S. Open title after announcing Friday he will sit out the rest of the 2017 season in order to recover from a knee injury.

The 32-year-old Wawrinka wrote on his Facebook page that he underwent knee surgery and will not return until 2018. On Wednesday, he announced he was pulling out of the upcoming Rogers Cup in Montreal and the Western & Southern Open in Cincinnati.

"I am sad to announce that after talking with my team and doctor I had to make a difficult decision to undergo a medical intervention on my knee," Wawrinka wrote on Facebook. "This was the only solution to make sure I will be able to compete at the top level for many more years. This is obviously extremely disappointing, but I'm already looking ahead and planning my recovery.

"I love this sport and I will work hard to get back to my top level and play many more years. I also want to take this opportunity to thank my fans for sending plenty of messages of support during the last couple of days. I will see you all in 2018!"

The three-time Grand Slam champion struggled with his play during the grass-court swing at the Aegon Championships and Wimbledon. He had to ice his knee during his first-round loss to Russia's Daniil Medvedev at Wimbledon.

Wawrinka finished the year with a 26-11 match record, including one ATP title at the Geneva Open. He also reached the finals of the BNP Paribas Open (losing to Roger Federer) and at the French Open (losing to Rafael Nadal).

Novak Djokovic, ranked No. 5, also will also miss the U.S. Open after announcing on July 26 that he will not return until 2018 as a result of a right elbow injury. Wawrinka defeated Djokovic in last year's final.