The 17-time Grand Slam champion, who has slipped to No. 7 in the rankings, was playing in his first tournament since losing in the fourth round of the U.S. Open.
Federer rallied from a break down in the second set and 5-3 down in the tiebreaker, but couldn't recover after being broken early in the deciding set.
Monfils, who had a 1-6 record against Federer coming into the match, next plays the winner between top-seeded Novak Djokovic and Fabio Fognini of Italy.
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