Novak Djokovic's US Open tennis title defence is over after the world No.1 succumbed to a shoulder injury in his round of 16 clash with Stan Wawrinka.
With the Swiss leading 6-4 7-5 2-1, Djokovic decided he could not continue after dropping his serve for the fourth time in the match.
The top-seeded Serb has been troubled by a painful left shoulder during the past week at Flushing Meadows and appeared to be uneasy during the one hour and 46 minutes match on Arthur Ashe Stadium.
"It is never the way you want to finish a match. He is an amazing champion and we had amazing battles in the past," Wawrinka said in his on-court interview.
"It has really been tough since my surgery. It feels amazing to be back at this level."
It is Djokovic's earliest exit at the New York grand slam since a third-round loss in 2006 to Australia's Lleyton Hewitt.
"I don't want to talk about my injuries," Djokovic told reporters. "I've said that in the past. I am sticking to that."
Wawrinka, who beat Djokovic in the 2016 US Open final, advanced to a quarter- final clash with in-form Russian Daniil Medvedev.
The fifth-seeded Russian came from a set down to knock off German qualifier Dominik Koepfer 3-6 6-3 6-2 7-6 (7-2) and reach his maiden grand slam quarter-final.
The New York crowd, angered by what tournament organisers called Medvedev's 'visible obscenity' two nights earlier, were firmly behind Koepfer when he went a set and a break up.
But Medvedev kept his cool and eventually prevailed before again goading the crowd with his celebration, although this time the reaction from those at Louis Armstrong Stadium was more mixed.
"You give me so much energy to win - thank you," he said. "You are the best!"