Reigning US Open champion Stan Wawrinka will miss his title defence to have knee surgery and will sit out the rest of the year, the world number four announced.

The three-time Grand Slam champion had already pulled out of the upcoming ATP tournaments in Montreal and Cincinnati, and joins last year's US Open runner-up Novak Djokovic on the sidelines for the remainder of 2017.

"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 on Facebook.

"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 will see you all in 2018!"

The 32-year-old Wawrinka, like Djokovic, will be hoping the time off can help him come back revitalised after a difficult month that included a first-round exit at Wimbledon.

On Thursday, Wawrinka admitted that the knee injury had hampered him during that shock defeat to unseeded Russian Daniil Medvedev in London.

The four-set victory over Djokovic in the 2016 final at Flushing Meadows gave Wawrinka his third major crown, taking him level with current World No.1 Andy Murray, who has been struggling with a hip injury and hasn't played since Wimbledon.

This season Wawrinka has had fluctuating fortunes, but did reach his second French Open final where he was thrashed by Rafael Nadal.

He will be hoping to emulate the Spaniard and his legendary fellow Swiss Roger Federer, after both returned from lengthy absences to claim Grand Slam titles this season.

Federer, who turns 36 next week, also did not play last year after Wimbledon, but has won both the Australian Open and Wimbledon this year and will be looking for a 20th major title in New York when the US Open gets underway on August 28.

With Wawrinka fourth in the world rankings, one place above Djokovic, 2014 champion and sixth-ranked Marin Cilic is likely to be the fourth seed, a month after finishing runner-up at Wimbledon.

However, Cilic, who has an adductor injury, is skipping the Montreal Masters next week as is Murray.

Because of the lost ranking points, Wawrinka will definitely slip out of the world's top eight by the start of next year, and could be as low as 11th.