Birthday boy Monfils beats Vallejo in first round of his last US Open

French showman Gael Monfils has celebrated his 40th birthday with a 2-6 6-1 6-2 6-0 win over Adolfo Daniel Vallejo to reach the second round of the US Open on Tuesday, keeping his farewell Grand Slam campaign alive.

A day after fellow veteran Stan Wawrinka played the final Grand Slam match of his career, Monfils showed he was not ready to follow him out the door yet, drawing huge applause from fans on Louis Armstrong Stadium as he fought back from a slow start.

Monfils also became the oldest player to win a main-draw singles match at Flushing Meadows since Jimmy Connors defeated Jaime Oncins on his 40th birthday in 1992.

There was no cake on court, but Monfils received a rendition of Happy Birthday midway through his speech.

"That was very special. Playing on my 40th birthday ... you guys made it incredible," Monfils said.

"You guys made it electric, the atmosphere was unbelievable. I tried to put on a great show for you."

A New York semi-finalist in 2016, Monfils produced an erratic display to drop the first set but roared back in the second to level the match.

Monfils dominated the third set and fired a crosscourt forehand winner to clinch it, as fans rose to their feet in anticipation of yet another electrifying win from the former world number six.

Paraguayan Vallejo freely surrendered breaks in the fourth set and wild card Monfils fed off the crowd's energy to secure the victory with a big ace to set up a meeting with American Learner Tien.