Sinner, the No.1 seed, moved into his fifth final of the season with a 6-2 6-2 defeat of Lloyd Harris in Antwerp.
The 20-year-old Italian, who is in the hunt for a maiden ATP Finals spot, dominated Harris from the baseline and saved all three break points he faced.
There were only 16 unforced errors from the racket of the composed Sinner, who will be bidding to win his fourth title of the season on Monday (AEDT).
Sinner said: "I am very happy to be in the final. I just love playing here and love playing indoors, so hopefully I can play a great match again tomorrow."
Schwartzman ended 20-year-old American Jenson Brooksby's impressive run with a 6-4 6-0 victory.
The Argentinean struck 15 winners and secured five breaks as he marched into the championship match.
Russian Aslan Karatsev, the No.2 seed, will come up against Marin Cilic in his first final on home soil after knocking compatriot Karen Khachanov out of the Kremlin Cup.
Karatsev beat Khachanov 7-6 (9-7) 6-1, while No.6 seed Cilic – a two-time winner of the tournament in Moscow – got the better of Ricardas Berankis 6-3 6-4.