Read: Nadal bullish on season-ending success
Monfils served so quickly in the second set the ball boys barely had time to get into position after each point before the ball was back in play.
Television commentators said at one stage they thought Monfils was in a hurry because he had a tee time booked to play golf.
During one rally he turned his back on his opponent briefly, prompting disbelief in the commentary box.
"To not give 100 percent and be so blatant about it, I think it's disrespectful," two-times U.S. Open champion Tracy Austin said on Tennis Channel.
"It looked like exhibition tennis. We've seen that way too many times with him.
"With Gael it's so difficult to watch because you know his upside ceiling could be so big but he just goes away and doesn't even try."
Despite his apparent indifference, Monfils actually had a set point in the second set before succumbing.
World number one Novak Djokovic, who had a first round bye, will face Frenchmen Benoit Paire in the second round.
France's 10th seed Gilles Simon was also sent packing on the first day after losing to Croatia's Ivo Karlovic 6-4 6-7(3) 6-3.
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