Grigor Dimitrov was playing sublime tennis on Monday and the world No.1's five-set record would not have encouraged him, Sinner having lost six, won one.
Two games later Dimitrov, having just served an ace to tie up the third set 2-2, slumped to the ground holding the pectoral muscle on the right, serving side of his chest.
It looked bad, and it was. Sinner helped his stricken opponent to his feet, the medics attended to the Bulgarian, he briefly stepped off court, then returned and conceded.
The 34-year-old was in so much discomfort he had to use his left arm to support his right arm when he shook hands with the chair umpire, grimacing as he did so.
This was the fifth grand slam in succession in which Dimitrov has been forced to quit through injury, and he was distraught.
And Sinner did not seem like a man who enjoyed his reprieve.
"I don't take this as a win at all, this is just an unfortunate moment for us to witness," said the top seed after advancing with the scores 3-6 5-7 2-2.
"He has struggled in grand slams with injuries a lot, so seeing him again with this injury is very tough.
"He has been so unlucky in the past couple of years. He is an incredible player who is a good friend of mine. We understand each other very well off the court.
"Seeing him in this position, if there was a chance for him to be in the next round he would deserve it.
"This is not the end we wanted to see and it's very sad."
Sinner, nevertheless, is into the last eight and a meeting with Ben Shelton, who had seen off another Italian, Lorenzo Sonego, 3-6 6-1 7-6 (7-1) 7-5 earlier on Monday.
Sinner will hope his elbow complaint, which happened when he braced a fall with his right hand while still holding his racquet, is not serious because Shelton has looked in powerful form.
By reaching the last eight, the American beat his watching father's best at Wimbledon, Bryan Shelton having been stopped in the fourth round in 1994.
"He was a serve-and-volleyer. Big serve. Came forward all the time. He would like to see me coming forward a little bit more than I am. My argument is I think I'm better than him from the baseline," Shelton said.
In the other quarter-final in this section of the draw, Novak Djokovic, who beat Australia's Alex de Minaur in four sets after losing the first 6-1, will meet Flavio Cobolli.
The Italian defeated Marin Ciic 6-4 6-4 6-7 (4-7) 7-6 (7-3) on No.2 court.
Cobolli, who like Shelton is coached by his father, said: "I'm real emotional because this is the day that I never forget."
Talking of Cilic, a former US Open winner and Wimbledon finalist, Cobolli added: "Everyone on my family, on my team, always told me that I live for this kind of match.
"I think is true because I really love to play against a big player, against a legend of our sport. I like to play on this kind of stage, this kind of tournament."
Next up is Djokovic on a show court. They don't come much bigger than that.
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