Monfils, the third seed, got the better of world number 12 and second seed Simon in a match that lasted two hours and 25 minutes.
It was a great comeback from Monfils who had to save a match point in the second set.
Simon also missed a total of eight break point opportunities in the second set.
It was the third career meeting between the players and the first one that Monfils has won.
Monfils will take on top seed Tomas Berdych in the final after a 6-3 6-4 victory over seventh seed Philipp Kohlschreiber.
A solitary break in both sets proved decisive for the Czech as he completed victory in 82 minutes.
