Novak Djokovic's US Open title defence came to an end after he was forced to retire injured when trailing by two sets to Stan Wawrinka.
The crowd on Arthur Ashe were stunned when the world No 1 approached Wawrinka and embraced him with the scores 6-4 7-5 2-1.
Djokovic had struggled with a left shoulder injury all week, although it was not apparent during the opening two sets with Wawrinka reprising the sort of form which made him the champion in 2016.
But the Serb took a medical time-out after dropping the second set, and two games into the third he decided enough was enough, which sparked boos from the Flushing Meadows crowd.
It means Wawrinka will face fifth seed Daniil Medvedev in the quarter-finals, while the draw has opened up for Roger Federer, who was in imperious form against David Goffin and had been expected to run into Djokovic in the last four.
"I'm sorry for the crowd. Obviously they came to see a full match, and just wasn't to be," Djokovic said. "I mean, a lot of people didn't know what's happening, so you cannot blame them."
He explained he had been "taking different stuff to kill the pain instantly. Sometimes it works, sometimes it doesn't.
"The pain was constant for weeks now."
Djokovic had won 36 of his past 37 Grand Slam matches, and four of the last five major titles, in one of the most dominant stretches this sport has seen.
That had pushed his Slam trophy total to 16, moving within four of Federer's record 20, and within two of Rafael Nadal's 18.
He also had been 11-0 in fourth-rounders at Flushing Meadows.
"It's never the way you want to finish the match," said Wawrinka. "I feel sorry for Novak."