Milos Raonic took less than 90 minutes to secure the victory over his French opponent, coming through 6-3 6-4 to join Novak Djokovic on one win apiece in the Ivan Lendl group.
The Canadian made a quick start to the match and soon raced into a 3-0 lead courtesy of a break in just the second game.
That proved to be a decisive moment in the opener as neither player threatened another break throughout, allowing Raonic to serve it out and take it 6-3 with his first set point.
Monfils was forced to save four more break points in his opening two service games of the second set too, and he eventually succumbed in the seventh game as Raonic took complete control of the match.
The world number four then wrapped up the victory with his second match point to get his tournament off to a winning start.