Alcaraz brushed aside the remaining home player, Briton Cameron Norrie, 6-2 6-3 6-3, on the same Centre Court on which less than 24 hours earlier Sinner had escaped when two sets down after Grigor Dimitrov had to retire with a chest muscle injury.
Amid the drama surrounding the Bulgarian on Monday night (local time) it was largely forgotten that Sinner had himself been nursing an injury to his elbow, incurred while breaking a fall, and had taken a medical time-out for it.
On Tuesday morning Sinner, who is due to meet American Ben Shelton in the quarter-final on Wednesday, had an MRI scan, and in the afternoon he cancelled his planned practice session.
There was no official update from Sinner's camp but his Australian coach Darren Cahill reportedly told ESPN the Italian had a hit indoors for 20 to 30 minutes.
"It was quite an unfortunate fall," Sinner said on Monday night. "I felt it quite a lot, especially on serve and forehand. We are going to check with MRI to see if there's something serious, and then we'll try to adjust it."
There were no such worries for Alcaraz. He lost the first three points on serve, but prevented Norrie securing the break and never looked back. He took the first set in 28 minutes and the match in 99.
The victory extended his winning streak to 23 matches and his record on grass to 34 wins from 37.
"To be able to play another semi-final here at Wimbledon is really special," Alcaraz said. "I am really happy with the level I played today against a really difficult player like Cam."
Fritz's path to the last four was not as smooth as the Spaniard's. Russian 17th seed Karen Khachanov won the third set 6-1, the American needed a medical time-out, and there was another technology malfunction with the automated line calls.
"The match was going so well for me for two sets," he said after taking a fourth set tie-break to wrap up the match 6-3 6-4 1-6 7-6 (7-4).
"I've never had a match just flip so quickly, so I'm really happy with how I came back in the fourth set and got it done.
"I felt I couldn't miss and then all of sudden I'm making a ton of mistakes.
"Momentum was definitely not going to be on my side going into a fifth."
Fritz said the treatment on his right foot ahead of the fourth set was just a minor matter.
"It's totally fine, it's pretty common, a lot of players do this tape job so your foot doesn't get irritated," he said.
"I think I ripped it off at some point in the second so I just needed to get it re-done."
The erroneous line call came soon after when 'Fault' was incorrectly called after a Fritz backhand landed well inside the baseline.
It became evident the system was still tracking the initial serve so chair umpire Louise Azemar-Engzell ordered the point be replayed.
The All England Club explained the system had failed to reset because the ball from Fritz's first serve was still being retrieved when he started lining up his second.
