By Julie Glass

Novak Djokovic, the No. 1 player in the world and the U. S. Open defending champion, suddenly retired during the third set of his fourth-round match against Switzerland’s Stan Wawrinka at Arthur Ashe Stadium in New York due to a left shoulder injury.

Wawrinka, the No. 23 seed, had grabbed an early lead when Djokovic called the match. Wawrinka had won the first two sets 6-4 and 7-5, respectively, and was up 2-1 early in the third set when Djokovic approached the net, clearly ready to be done.

“Very frustrating,” Djokovic said,via the New York Times. “Obviously not the first, not the last player to get injured and to withdraw from one of the biggest events in sport. But obviously, I just came off the court, so of course it hurts.”

Djokovic was met with a mix of applause and boos as he walked off the court too, and gave a thumbs up to the crowd before entering the tunnel.

“Look, I’m not being offended by, you know, mistreated by anybody,” Djokovic said when asked about the crowd reaction, via the New York Times. “I don’t really pay too much attention on that. You know, I like to respect others. I hope that others can respect me and my decision.

“I’m sorry for the crowd. Obviously they came to see a full match, and just wasn’t to be. That’s all it is. I mean, a lot of people didn’t know what’s happening, so you cannot blame them.”

Djokovic had battled through pain in his left shoulder during his first three rounds at the US Open, too, especially in his second-round match with Juan Ignacio Londero.