The U.S. Open's defending champion and No. 1 seed has been eliminated. . Djokovic retired due to an injured shoulder on Sunday night. . He was trailing Stan Wawrinka 6-4, 7-5, 2-1 in the match. . The pain was constant for weeks now, Novak Djokovic, via statement . The crowd booed Djokovic as he left the court, likely due to the shortened match time. . I'm sorry for the crowd. Obviously, they came to see a full match, and just wasn't to be. I mean, a lot of people didn't know what's happening, so you cannot blame them. , Novak Djokovic, via statement. Djokovic had won 36 of his past 37 Grand Slam matches and four of the past five major titles. . He has 16 career Grand Slam titles, four shy of Roger Federer's record of 20.