Yet the Spaniard was still able to win the battle with his own mind to earn his 100th tour clay-court victory and advance to the French Open quarter-finals.
The 22-year-old subdued dangerous Ben Shelton 7-6 (10-8) 6-3 4-6 6-4 on Court Philippe Chatrier on Sunday, meaning he's now lost a set in each of the last three matches of his title defence.
They've all featured spells of familiar stellar quality but also careless patches - "dumb mistakes" as Alcaraz dubbed them - that have begun to irritate him even if, in truth, they've never looked like derailing him.
Still, just as he laboured a little to get past Damir Dzumhur in the previous round, there was evidently a slight feeling of relief for the 22-year-old that he had at last managed to get past the booming US leftie Shelton in three hours 19 minutes.
"Honestly, today I fought against myself in the mind," said Alcaraz.
"I just tried to calm myself - in some moments I was mad.
"I was angry with myself, talking not good things. I'm happy that I didn't let that thought play against me.
"I tried to calm myself and tried to keep going."
He did keep going and the crowd couldn't have loved him more, especially when he owned up that one seemingly amazing winner he'd hit shouldn't count.
He had flung his racquet out at full stretch to get to a Shelton passing shot and, miraculously, pulled off what looked to everyone like a winning volley. He was awarded the point, but admitted to the chair umpire that his hand wasn't holding the racquet and got a rousing ovation from the crowd for his sportsmanship.
"I would have felt guilty if I didn't say anything," Alcaraz explained.
"It's just about the respect we have against each other."
Alcaraz, after 19 wins in 20 clay-court matches this season now, will be hot favourite to get past his next obstacle too, even though 12th seed Tommy Paul looked a potentially troublesome last-eight foe as he outclassed Australian Alexei Popyrin 6-3 6-3 6-3.
The first American man to reach the French Open quarters in 22 years since Andre Agassi, Paul loves pitting himself against the Spaniard and had beaten him a couple of times, though Alcaraz won their last encounter at Roland Garros in the Olympic quarter-final.
"I remember that every match I've played against him he was really tough," said Alcaraz.
"In grand slams, Masters 1000s, all tournaments I've played against him it wasn't easy at all. His level is really high right now.
"I'll try to keep going, trying to be better. I'll try to learn from the mistakes I made today. I'm not going to make the same against Tommy."
Frances Tiafoe was another American to progress, defeating Daniel Altmaier, the German who had knocked out his US compatriot Taylor Fritz, 6-3 6-4 7-6 (7-4).
Tiafoe's next opponent will be Italian No.8 seed Lorenzo Musetti, who beat 10th seed Holger Rune 7-5 3-6 6-3 6-2 in a match that ended after midnight.
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