With Djokovic out, the remaining players can feel fortified that they will not have to face the best player on tour. One of those beneficiaries is No. 3 Roger Federer, who lost to Djokovic in a thrilling Wimbledon final in July. Federer, who has not won the U.S. Open since 2008, beat No. 15 David Goffin earlier on Sunday and will play unseeded Grigor Dimitrov in a quarterfinal.

He now has a path through the Open that no longer includes the possibility of facing Djokovic in the semifinals.

But Wawrinka has shown that he, too, must be seen as a dangerous opponent, a smart, powerful battler. He served extraordinarily well Sunday, winning 84 percent of the points on his first serve and was only broken once.

“Most important is the way I’m playing, the way I’m moving,” Wawrinka said. “Tonight I think was really high level.”

The same was not the case for Djokovic, who would not give details of his injury, other than to acknowledge that it was his left shoulder. Before the tournament began he said the shoulder was bothering him on backhands and on the toss of his serve.

The injury probably accounted for many of his 35 unforced errors in the abbreviated match. He was unable to practice the day after his first-round match, but it seemed to subside in his next two matches. There were no indications that he was in distress, until his poor play Sunday night.

Wawrinka said he sensed something was wrong with Djokovic, and assumed it was his balky shoulder, but was playing so well he did not change his approach. He also had a good deal more empathy for Djokovic than the fans did.