Playing on his 40th birthday, Monfils opened shakily against Adolfo Daniel Vallejo of Paraguay, but decisively turned the tables after the first set for a 2-6, 6-1, 6-2, 6-0 win.
Monfils, who will retire after this season, played with his trademark showmanship, offering exaggerated bows after embellishing a putaway shot in the fourth set with a leap in the air.
Evidence of approval came when the crowd joined an impromptu performance of "Happy Birthday" after the match.
"You guys always make it electric, amazing," Monfils said in an on-court interview. "Let's keep it going for the second one."
This is Monfils' 18th appearance at the US Open. His best showing came in 2016 when he reached the semi-finals and lost to Novak Djokovic.
Monfils, who reached a career-high ranking of sixth in 2016, is down to 311th, having only played a handful of tournaments in 2026.
In contrast to Monfils, who is playing in his 70th Grand Slam tournament, Tuesday marked just the third of the 22-year-old Vallejo.
US Open 2026
The US Open is the fourth and final Grand Slam of the tennis season, running from August 30th to September 13th. It is a hard-court tournament held at Flushing Meadows in New York City. Follow the links below for more information!
All you need to know | Men's singles draw | Women's singles draw | Latest US Open news