The 10th-ranked Monfils converted three of nine break points against the hard-serving Canadian. He's looking for his first title of the year.
Nieminen beat American player James Blake 7-6 (5), 5-7, 6-2 to reach his third Stockholm Open final.
Blake, who beat 15th-ranked Juan Martin del Potro before getting a walkover into the semifinals after David Nalbandian pulled out with an injury, double-faulted seven times and struggled with his serve.
Nieminen converted three of his five break points in the third set, giving the 73rd-ranked Finn a third chance to win his first title at the Royal Tennis Hall.
In his 12 Stockholm Open appearances, he has reached the semifinals five times, and was a finalist in 2001 and 2006.