The third-seeded Frenchman, looking to make it two trophies in as many weeks following his triumph in Montpellier, fired 14 aces and 35 winners to beat Krajinovic.
Monfils squandered four match points at 5-4 in the second set, but the 33-year-old held his nerve in the tiebreak and will look to lift the 10th title of his career in Sunday's final.
If he prevails, he will become the first man since Robin Soderling in 2011 to win back-to-back titles in Rotterdam.
Earlier, 19-year-old Auger-Aliassime stormed into his fourth ATP final with a 7-6 (2), 6-4 victory over Spain's Pablo Carreno Busta to keep alive hopes of a maiden crown.
"The first set was really tight with both of us serving well. I was serving the best I've ever served in that first set and then played a solid tiebreak," the Canadian said.
After edging the first set, Auger-Aliassime got the decisive break in the third game of the next set and held serve at 4-3 to quell a late comeback attempt by Carreno Busta.
The world No. 21, who saved all four break points that he faced, closed out the match with his 16th ace.
"In the second set, things got interesting with that long (third) game. I was thinking: 'If this goes my way, it might be tough for him.' So, I really pushed to try to get through," Auger-Aliassime added.
"It wasn't easy. I came up clutch again, so obviously I'm happy with myself."