The Spanish four-times 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.
"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.
Andrey Rublev was close to forcing a third set against Carlos Alcaraz in a lengthy tiebreak. (AP PHOTO)
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 Norway's Casper Ruud.
After sharing the opening six games Alcaraz broke Rublev's serve with a scorching forehand winner and reeled off five successive games to seize control of the match.
World No.8 Rublev, who lost his opening match to Alexander Zverev, managed to stay with Alcaraz in the second set and could have earned a couple of break points at 2-2 had he not missed a routine ball after chasing a drop shot.
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.
Ruud will face world No.2 Zverev later on Wednesday with both players seeking their second win. Should Ruud win, he would guarantee himself a semi-final berth while Rublev would be eliminated.
In the doubles Matt Ebden and his Indian partner Rohan Bopanna are out after losing to Marcelo Arvevalo and Mate Pavic 7-5 6-3.Â
With German duo Kevin Krawietz and Tim Putz defeating home pair Simone Bolelli and Andrea Vavassori 7-5 6-4, Ebden and Bopanna can no longer qualify even if they win their final match against Krawietz and Putz.
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