Roger Federer's hopes of another US Open title have been given a massive boost after defending champion and No 1 seed Novak Djokovic bowed out this afternoon.

Djokovic's title defence ended abruptly when he stopped playing during his fourth-round match against Stan Wawrinka because of pain in his left shoulder.

Djokovic was trailing 6-4, 7-5, 2-1 and being thoroughly outplayed when he retired from the match in Arthur Ashe Stadium, shaking his head as he walked over to the chair umpire to say he was conceding.

"It's never the way you want to finish the match," said Wawrinka, who will face No 5 seed Daniil Medvedev in the quarterfinals. "I feel sorry for Novak."

It promises to make Federer's path to a highly sought after first US Open final against Rafael Nadal much easier after the Swiss star was scheduled to meet Djokovic in the semis.

A fiery Federer has his groove back and he proved it by destroying David Goffin to advance to his 13th US Open quarter-final. After surprisingly dropping the opening set in his first two rounds, the Swiss cruised through his second-straight easy victory by belting Goffin 6-2 6-2 6-0 in less than 90 minutes, to set up a meeting with Grigor Dimitrov.

Make it through that, and he won't have to play Djokovic, who had won 36 of his past 37 Grand Slam matches, and four of the last five major titles, in one of the most dominant stretches this sport has seen.

But he began complaining about his left shoulder during a second-round victory, when he repeatedly got massaged by a trainer during changeovers.

Djokovic won his next match, looking good, and declared himself mostly pain-free, although he refused to disclose any details of the injury or what type of treatment he had received.

His bid for a fourth U.S. Open championship and 17th Slam trophy in all suddenly dissipated at the conclusion of what for him amounted to a listless and ineffective effort.

Against Wawrinka, a three-time major champion himself, Djokovic never quite had the usual verve on his shots or range on his formidable service returns. He was out of sorts on all manner of shots, accumulating 30 unforced errors and only 12 winners through the first two sets.

He managed to lead 3-0 and 4-1 in the second set, but that was just about all he had in him.When that set ended, Djokovic had a trainer on for a massage, but soon thereafter, his title defence was over.

Djokovic entered the night with a 19-5 head-to-head edge against Wawrinka across their careers. This, though, was their first meeting since the 2016 U.S. Open final, won by Wawrinka.

This scuttles the possibility of a semifinal between Djokovic and 20-time major champion Roger Federer, which would have been a rematch of their historic Wimbledon final in July.

Djokovic won that one in an unprecedented fifth-set tiebreaker after nearly five hours.