Simon brushed past his compatriot with some brilliant and assured play as he clinched the straight-sets victory in 84 minutes.
The 27-year-old stormed through to his sixth ATP World Tour semi-final of the season as he broke serve five times from eight opportunities, taking Monfils’ first serve apart with ease.
"It was difficult to win against him," said Simon. "I always try everything to win, even if (Monfils) was not in his best shape.
“I think it was tough but I played him the right way. My game is fine. It’s difficult to say something about it today because I don’t really play my game with Gael.
“I've known him for a long time, so I know the way I have to play him and I think I will have to play another [style of] game tomorrow."
In the semi-finals, Simon will face top seed Janko Tipsarevic, who earned his 50th match win of the season as he battled past fifth-seeded Spaniard Fernando Verdasco 7-6(5), 6-4 in one hour and 50 minutes.
The Serb saved seven of the nine break points he faced and broke Verdasco three times as he improved to a 4-2 head-to-head lead over the Madrid native.
Another Frenchman, Richard Gasquet, sealed his place in the semi-finals as he beat Bernard Tomic of Australia 7-6 6-4 in what was a very tight encounter.
Meanwhile, Jarkko Nieminen of Finland beat talented Canadian Milos Raonic 6-3 7-6 to progress through to the last four, where he will meet Gasquet.