Tien, seeded 14th, defeated Monfils of France 6-3, 6-4, 6-3.

The Frenchman, who has said 2026 will be his last year on the tour, stayed with Tien in all three sets, but didn't win enough of the key points to seize control.

The unflappable Tien, a lefty skilled at moving opponents around the court and finishing with a big forehand or touch shot, saved all eight break points over the two hour and 16 minute match.

But the American rising star was not the pick of the Big Apple crowd.

Monfils has amassed a sizeable following in New York since his US Open debut in 2005, even when playing an American.

A video montage of Monfils highlights shown after the contest consisted of some of the Frenchman's greatest hits, including leaping smashes and unbelievable gets.

During the match, chants of "Monfils" erupted frequently, most loudly in the third set.

"It's been a long ride," Monfils said as he thanked the crowd.

"I felt all the love this year," Monfils said. "You are my second best crowd. Of course, it can't top Paris, but you are up there!"

Thursday marked Monfils 18th appearance at the New York Grand Slam, a run that began with a five-set first-round loss to Novak Djokovic of Serbia in 2005 when both men were teenagers.

His best showing came in 2016 when Monfils reached the semi-finals, again losing to Djokovic.

Tien, 20, is one of a handful of American men positioned for potentially deep runs in New York this year.

Although Tien did not have the crowd during the contest, he received warm applause after the match, which concluded with a warm handshake and embrace with a smiling Monfils after match point.

"I really didn't want the greatest comeback of all time to happen against me," Tien said. "It's very special to win in your home country."

Tien will next play 17th-seeded Jakub Mensik of the Czech Republic.