After defending champion Sinner ruthlessly subdued 136th-ranked qualifier Terence Atmane 7-6 (4), 6-2, Spanish second seed Alcaraz defeated an ailing Alexander Zverev, who was suffering badly from the 32 Celsius heat and humidity, by 6-4, 6-3.
Alcaraz and Sinner have played for trophies this season in Rome, Roland Garros, and Wimbledon, with the Italian winning their most recent at the All England Club last month.
Alcaraz will also be playing in his second Cincy final after losing to Novak Djokovic in 2023.
The Spaniard increased his ATP season match win lead to 53 in a year of five titles.
Alcaraz broke Zverev once in the opening set to claim it before the German, who is diabetic, began feeling poorly and took a medical timeout off court after the third game of the second set.
Zverev, who has retired in only two matches since 2014, came back out to finish what was a patchy match from Alcaraz, who double-faulted four times in the second game of the second set but won the last 12 points with Zverev running on fumes.
"We started well with good rallies, a good level," Alcaraz said. "All of a sudden, he felt bad and I was thinking more about how he was feeling instead of playing good tennis."
"It was tough and I just wish him all the best."
Alcaraz said he is keen to try and take his Wimbledon revenge on Sinner in the unorthodox Monday final.
"We always bring our best tennis. We raise each other's level. I'm ready to take the challenge," Alcaraz said.
"I will try and adjust my game better and correct what I did wrong in our last match. I want to be ready with my 100%. Mentally, I'll be ready - I'm excited for Monday."