Novak Djokovic’s left shoulder was aching, his chances were fading in the fourth round against Stan Wawrinka, and, soon enough, the US Open’s defending champion and No. 1 seed was out of the tournament, leaving to a chorus of boos.

Djokovic shook his head as he walked over to the chair umpire to say he was retiring from the match because of that shoulder while trailing 6-4, 7-5, 2-1 and being thoroughly outplayed Sunday night.

Some spectators booed as Djokovic walked off the court in Arthur Ashe Stadium to head toward the locker room, his two equipment bags over his other shoulder.

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He responded with a thumbs-up.

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