Defending champion Carlos Alcaraz survives a scare from Ugo Humbert as he moves on to the Wimbledon quarterfinals. (1:00)
Alcaraz beat Sinner in five sets in the French Open semifinals in June en route to his first title at Roland Garros, and the two would meet again in the last four if they win their next matches.
On Sunday, Alcaraz seemed to have his match all but won when he clinched the second set by chasing down several seemingly out-of-reach shots on set point, even falling over after hitting one forehand and quickly getting back up to get to the next ball.
Asked to describe his play on that point, Alcaraz just said: "Unbelievable, I guess."
But Humbert nearly staged a surprising comeback and threatened to take the match into a fifth set when he held three straight break points for a 5-3 lead in the fourth.
Alcaraz came back to win that game with the help of a couple of aces then broke for a 6-5 lead by hitting a deep forehand winner.
He set up match point with one of his delicate forehand drop shots and converted it with a service winner.
"I felt great playing today. I think I played at a really high level," said Alcaraz, who will next play No. 12 Tommy Paul.
Shelton started aggressively against Sinner under the Court 1 roof by serving and volleying in a bid to unsettle the Australian Open champion.
Sinner was equal to the task, however, and broke the feisty American twice to comfortably pocket the opening set.
The Italian player consolidated an early break in the second set with a booming ace and pulled away for a two-set advantage, as Shelton was left with another battle on his hands having been dragged the distance in each of his last three rounds.
Shelton raised the hopes of fans by breezing ahead 4-1 in the third set. But Sinner came back and held for 5-5 after wrapping his racket behind his back and hitting the ball between his legs to get it over the net then followed it up with a forehand passing shot to clinch the game.
"That was just luck," Sinner said his shot.
Sinner saved one set point at 6-5 and three more in the tiebreaker before converting his second match point when the hard-serving Shelton double-faulted.
The and contributed to this report.
