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The victory completed a stunning comeback from Canadian Auger-Aliassime, who lost his first three sets of the week to slip towards an early exit at Inalpi Arena.

But a comeback victory against Ben Shelton kept in on course, and no doubt filled him with the confidence to dispatch Zverev and move through to the final four.

“This is a high-value tournament for players,” Auger-Aliassime said.

“It’s like a grand finale and when you have a look at the list of champs there have been many No.1s.

"You want to be in the final but I’ll have to go through a great player to do that. I will take my chance if I have it.”

Auger-Aliassime struck late in the first set to break Zverev, despite the German saving three set points from 4-5, 0/40.

The second set was equally difficult for both players as neither could get a foothold.

Zverev, though, looked the more likely to break mentally as he frequently gestured to his team in the stands, and when the tiebreak ensued, he eventually cracked.

From 5/4, Auger-Aliassime collected two minibreaks thanks to Zverev errors, claiming the match after two hours and seven minutes.

“It was a great first set. He was getting a few chances but I came up with big serves to save them,” Auger-Aliassime said.

“The first set was very solid and the start of the second set was, too. I had a lot of chances.

"When that happens you need to focus on the present but you’re aware of missed opportunities, so the match got a little tense from that moment. But I was able to hold serve and in the tie-break it was tight till the end.”

Auger-Aliassime is chasing his fourth ATP Tour trophy this year, having won in Adelaide, Montpellier and Brussels prior to the year-ending Turin tournament.

But his task will not be easy, with Alcaraz waiting the semi-finals and desperate to celebrate his world No.1 status  with one last tournament win for the year.