Fils secured a place in his first tour-level final since lifting the ATP 500 trophy in Tokyo in 2024 with an impressive straight-sets win over 2024 finalist Mensik.
The Czech was outstanding in his quarter-final win over Sinner but could not hit the same heights against Fils.
"It’s been a tough journey," said Fils, who is playing just his third event since returning after eight months on the sidelines with a back injury.
"Eight months without playing tennis, just watching the other guys battling... Staying in my bed and doing recovery was very long.
"My team knows how tough it was, but now it feels even better when I am coming back like this. To be back in a final, it means a lot."
Fils, ranked 40, recently added 2001 Wimbledon champion Goran Ivanisevic to his coaching staff and feels he may be in a position to beat Alcaraz, who has won both of their previous meetings which came on clay.
"If you believe, then maybe you can do it, so I believe 100 per cent," said Fils. "Let's see tomorrow on the court but for sure I am coming on court to fight."