Stan Wawrinka chose long-term health over a U.S. Open title defense.
The defending champion pulled out of the tournament Friday because of an injured knee, saying he decided to undergo a "medical intervention" and sit out the rest of 2017. He did not provide any specifics about the injury or treatment.
"This was the only solution to make sure I will be able to compete at the top level for many more years," Wawrinka said in a statement through his agency.
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He joins the man he beat in last year's final at Flushing Meadows, Novak Djokovic, in calling it quits for this season because of injury.
Like Roger Federer last year, Wawrinka couldn't make it to New York for the season's final major, which begins Aug. 28.
The fourth-ranked Wawrinka added last year's title to his 2014 Australian Open and 2015 French Open championships. He got back to the finals in Paris in June, losing to Rafael Nadal, but was eliminated by Daniil Medvedev in the first round at Wimbledon.
The 32-year-old Swiss was clearly bothered by his left knee then, icing it during changeovers. He said after the defeat the knee had been a problem on and off all season and he needed to figure out what was wrong.
Wawrinka finishes with a 26-11 record with just one title this season. He says he is looking toward his recovery and playing in 2018.
"I love this sport and I will work hard to get back to my top level and play many more years," Wawrinka said.
Wawrinka follows his former Olympic doubles and Davis Cup champion teammate in having to cut short his season because of his knee. Federer sat out the second half of 2016 but has returned strong this year, winning two majors and beating Wawrinka in the finals at Indian Wells.
Djokovic decided late last month he would miss the U.S. Open because of an injured right elbow, ending his streak of playing in 51 consecutive Grand Slam tournaments. He aims to return to the ATP Tour in January.
AP Tennis Writer Howard Fendrich contributed to this report.