The Spanish four-time grand slam champion has been suffering with respiratory issues in Turin and quit practice on the eve of the match after five minutes saying he was struggling to breathe.
But he had plenty of puff as, wearing pink nasal tape, he enjoyed a resounding 6-3 7-6 (10-8) victory on Wednesday night.
"I did surprise myself," Alcaraz said on court.
"I just tried to focus on my game and what I had to do and not that I wasn't feeling well. Once you step on court you have to forget that."
In the late match in the John Newcombe group, two-time former champion Alexander Zverev underlined his superb late-season form to beat Norway's Casper Ruud 7-6 (7-3) 6-3 for his second victory of the week.
Alexander Zverev returns a Casper Ruud ball during their clash in Turin. (AP PHOTO)
None of the eight men's singles matches so far at this year's season-ender have gone to three sets.
Should Germany's Zverev beat Alcaraz to win the group and Rublev defeats Ruud, three players would end up with one victory meaning sets and games would come into play to separate them.
Zverev was pushed hard by Ruud, who reached the final in 2022, but turned up the heat in the first-set tiebreak, banging down a couple of aces to consolidate a mini-break on the way to taking the opener.
Zverev then finished off the second set in 33 minutes to chalk up his Tour-leading 68th match win of the year.
"Today's match was better than the first match from the baseline," the world No.2 told Sky Sports.
Well, this was FUN rally ðŸŽâ€” ATP Tour (@atptour) Box office from @AlexZverev & @CasperRuud98 🎬#NittoATPFinals pic.twitter.com/Go7jkCcNoGNovember 13, 2024
The 27-year-old will now set his sights on Alcaraz, to whom he lost in the French Open final this year, although he did beat him at the Australian Open.
"We've played some very tough battles in grand slams this year, so I'm expecting a tough match," Zverev added.
"I'm looking forward to it and hopefully it will be fun to watch."
Third seed Alcaraz, bidding to win the title for the first time, looked sharper than during his opening group match which ended in a straight-sets defeat by Ruud.
After sharing the opening six games, Alcaraz reeled off five successive games to seize control of the match.
World No.8 Rublev, who lost his opening match to Zverev, managed to stay with Alcaraz in the second set.
Andrey Rublev was close to forcing a third set against Carlos Alcaraz in Turin. (AP PHOTO)
A high-octane conclusion to the match saw Alcaraz fend off two set points in the tiebreaker before sealing victory.
Rublev has now lost six successive matches at the ATP Finals in straight sets since reaching the semi-finals in 2022.
In the doubles, Australia's Matt Ebden and his Indian partner Rohan Bopanna are out after losing to Marcelo Arvevalo and Mate Pavic, 7-5 6-3.
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