Djokovic defeated the Frenchman 6-2, 6-1 in the last eight to extend his season record to 51 wins and just one defeat.
The 24-year-old, who has yet to lose a set on his way to the last four in Montreal, has now won all seven of his meetings with Monfils.
"I have played the best match of the tournament, no doubt," Djokovic said after his win over Monfils.
"I played really, really fast and tried to constantly put pressure on Gael because I didn't want to give him any time to get back to the match or to think about what he's going to play.
"Overall performance was fantastic."
Djokovic's semi-final opponent will be another Frenchman as he takes on Jo-Wilfried Tsonga.