Novak Djokovic was booed off the court after retiring from his US Open third-round match against Stan Wawrinka.

Djokovic called time on his tournament early in the third set after losing the opening two 6-4, 7-5.

"It is very frustrating. Of course it hurts that I had to retire," Djokovic said in The Guardian.

On the booing crowd, Djokovic said: "You cannot blame them."

Djokovic's exit means he has failed to retain his US Open title, with Rafael Nadal and Roger Federer now the favorites to win the tournament.

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Novak Djokovic was booed off court after retiring from his US Open third-round match against Stan Wawrinka.

The Serbian, who has won the tournament three times, withdrew from the match with a shoulder problem in the third set, having lost the opening two.

"I'm sorry for the crowd. They came to see a full match, but it wasn't to be," the world No. 1 said in The Guardian. "I mean, a lot of people didn't know what's happening, so you cannot blame them."

Listen to Djokovic get booed here:

—US Open Tennis (@usopen) September 2, 2019

Djokovic was dominated by Wawrinka in front of a capacity crowd at Arthur Ashe Stadium in Flushing Meadows in Queens, New York, with Wawrinka storming to take a 6-4, 7-5, 2-0 lead, at which point Djokovic decided he could not continue.

Last year's US Open winner had shown signs of his injury in the second round against Juan Ignacio Londero, where he needed treatment three times before eventually overcoming the Argentine in straight sets.

The problem seemed to have mostly subsided in his victory over the American Denis Kudla, thought it returned to haunt him against Warwinka, who is now set to face Daniil Medvedev in the quarterfinals on Tuesday.

"It is very frustrating. Of course it hurts that I had to retire," Djokovic said in The Guardian. "Some days the pain has been higher, some days with less intensity. Obviously I was taking different stuff to kill the pain instantly. Sometimes it works, sometimes it doesn't."

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Djokovic's exit paves the way for Rafael Nadal or Roger Federer to extend their advantages over him in the race to be the most decorated men's tennis player of all time.

Djokovic has won 16 titles at Grand Slam tournaments, while Nadal was won 18 and Federer 20.

Nadal is set to face Marin Cilic in the round of 16 on Monday, with Federer taking on Grigor Dimitrov in the quarterfinals on Tuesday.