Novak Djokovic’s U.S. Open title defence came unstuck on Sunday when a nagging shoulder injury forced the world number one to retire from his fourth-round match against Stan Wawrinka, prompting a chorus of boos from the crowd at Flushing Meadows. Wawrinka was leading 6-4 7-5 2-1 when Djokovic decided he could not continue, the pair embracing at the side of the court before the Serb walked off with his head down low but his left thumb stuck high in the air. “The pain was constant for weeks now. Some days higher, some days with less intensity, and obviously taking different stuff to kill the pain instantly,” said Djokovic, who has won the U.S. Open three times. “Sometimes it works, sometimes it doesn’t. “You just know when you know, I guess, when you feel like you’re not able to hit the shot anymore.” After dropping the second set, Djokovic immediately called for medical attention and sat courtside without his shirt on while the trainer gave him a massage. He returned to the court but the 16-times Grand Slam winner was still clearly hampered by the issue and after being broken to love following a double fault decided he could not go on. The shoulder injury nearly saw Djokovic retire from his second-round match on Wednesday but the 32-year-old seemed to have recovered by Friday, saying he was almost pain-free after winning in straight sets to reach the last 16. While frustrated in his bid to inch closer to Roger Federer’s all-time record of 20 career Grand Slam titles, Djokovic was confident there would be plenty more chances to add to his collection. “It’s a long road ahead hopefully for me,” he said. “I hope I can play for many more years. I’m planning to. I mean, I don’t see an end behind the corner at all. “Now it’s a matter of keeping my body and mind in shape and trying to still peak at these kind of events that are majors and that are the most significant in our sport.” Djokovic was off the pace in a lacklustre first set against three-times Grand Slam champion Wawrinka but seemed to right the ship in the second when he took a 4-1 lead. But a determined Wawrinka, with his serve clicking, fought his way back and twice held to love en route to taking the set for a 2-0 lead. Wawrinka, who had not faced Djokovic since beating him in the 2016 U.S. Open final, said he could see his opponent was struggling but did his best to focus on his own game. “For sure I could see some little thing that he was in trouble,” said Wawrinka, who will face Russian fifth seed Daniil Medvedev in the quarter-finals. “But I was most likely, most of the time, focussed on myself because I know how well he can fight. I know how well he can come back. Doesn’t matter how he’s feeling on the court, and that’s what I was focussing on.”