Number one seed Novak Djokovic exited the court to a chorus of boos Sunday night after retiring from the U.S. Open in the fourth round. His opponent, Stan Wawrinka, stormed to a 6-4, 7-5, 2-1 lead before Djokovic signalled that he couldn't carry on because of a shoulder injury. “The pain was constant for weeks now,” Djokovic explained afterwards. “Some days higher, some days with less intensity and obviously taking different stuff to kill the pain instantly. Sometimes it works; sometimes it doesn’t.” He apologized to a deeply disappointed audience, saying: “I'm sorry for the crowd. They came to see a full match but it wasn't to be.” Wawrinka, 34, will play Russian fifth seed Daniil Medvedev in the quarter-finals.