The injured Novak Djokovic retires injured to retire from the US Open, Stan Wawrinka enters the quarterfinals

NEW YORK: Novak Djokovic 's United States Open The defense of the title stopped on Sunday when the Serbian withdrew from his fourth round match against Stan Wawrinka due to a shoulder injury.

Wawrinka led 6-4, 7-5, 2-1 when world number one Djokovic, who won four of the last five Grand Slam titles, decided that he could not continue and left the Arthur Ashe Stadium field to a choir of boos.

After losing the second set, Djokovic immediately sought medical attention and sat on the court without his shirt on while the coach was massaging his shoulder.

He returned to court, but was still clearly hampered by the problem before calling him one day.

It 's never the way you want to finish a match, said Wawrinka. I am really sorry for Novak, he's an amazing friend.

The shoulder problem broke out earlier this week and he almost saw Djokovic knocked out in the second round on Wednesday, but the 32-year-old seemed to have recovered on Friday when he won in straight sets to reach the last 16.

I don't want to talk about my injuries, Djokovic told reporters. I said it in the past. I am sticking to that.

The next thing for Wawrinka will be the fifth Russian seeded Daniil Medvedev, who beat Sunday the German qualifier Dominik Koepfer on Sunday to reach his first quarterfinal of Grand Slam.