Stan Wawrinka, the 2016 US Open champion, has advanced to the quarter-finals in Flushing Meadows after Novak Djokovic retired due to injury.

The No. 23 seed broke for a 6-4, 7-5, 2-1 lead over World No. 1 Djokovic in the fourth round when the 16-time Grand Slam titlist retired due to a left shoulder injury.

“It’s never the way you want to finish a match. I feel sorry for Novak. He’s a friend and an amazing champion. We’ve played some amazing battles all my career,” Wawrinka said on court after the match. “I want to keep my level tonight. I was playing some very good tennis. I am happy to be back.”

Djokovic, who received treatment on his left shoulder after the second set, had not lost before the quarter-finals in Flushing Meadows since 2006. He was only 19 then.