In a gripping conclusion, the 23-year-old squandered four match points but held his nerve in a dramatic tie-break.

He eventually won 3-6 6-3 6-2 3-6 7-6 (7-5) in three hours and 57 minutes.

The Italian will play three-time champion Rafael Nadal for a place in Sunday's final after the Spaniard beat Diego Schwartzman 6-4 7-5 6-2.

"Right now I don't remember any points, just the match point," Berrettini said.

"What a great fight. I think it was one of the best matches I maybe ever saw - I was playing, but I also saw. I'm really proud of myself."

How an epic unfolded

Matteo Berrettini fell to the floor after sealing victory in his first Grand Slam quarter-final

Frenchman Monfils, 33, had gone a set and a break up as his bid to reach a first US Open semi-final since 2016 got off to the perfect start on Arthur Ashe Stadium.

Initially unable to settle, Berrettini gained in confidence and battled back to win 12 of the next 16 games and move a set from victory with Monfils looking increasingly fatigued.

The momentum flipped again in the fourth set as Monfils summoned the nerve to seize a fifth break point opportunity at 2-1 up - the single break of serve enough to ensure the match would go the distance.

In keeping with the flow of the contest, the pair exchanged breaks early in the fifth set - but it was the Italian who appeared to have mustered a final push for victory when he broke to love for a 4-2 lead.

That was not to be the end of the drama, as the enormity of the occasion struck Berrettini and he double-faulted on his first match point at 5-3. That quickly became break point as the experienced Monfils capitalised.

Holding to lead 6-5, two more match points were missed as Monfils forced a tie-break.

But despite threatening to let the opportunity slip once more the Italian, who had failed to progress beyond the first round in two previous attempts in New York, closed out to set up the biggest match of his career to date.