Spaniard Carlos Alcaraz defeated 2014 champion Marin Cilic 6-4, 3-6, 6-4, 4-6, 6-3 in one of the latest ever finishes at the U.S. Open on Monday to set up a next generation quarter-final clash against Jannik Sinner.

After battling for three hours and 53 minutes at Arthur Ashe Stadium, where the roof was closed due to rain, Alcaraz, 19, dropped to his knees in jubilation after sealing the contest on his first match point when Cilic's return hit the net.

The sizeable crowd on the main showcourt at Flushing Meadows gave the players a standing ovation with the clock striking 2.23 a.m., minutes shy of the latest finish at the hardcourt Grand Slam in New York.

Cilic, 33, seemed to have wrested the momentum back when he broke Alcaraz's serve late in the fourth set to level the contest at 2-2 and started the deciding stanza with another break in the opening game.

But Alcaraz, who thrilled the crowd with his athleticism during some breathtaking rallies, regained his composure to break Cilic in his next two service games and put himself on course for a third win this year against the Croat.

"Honestly, I have no idea," Alcaraz said on court when asked how he managed to clinch victory. "It was pretty, pretty tough at the beginning of the fifth set (being a) break down.

"Marin was playing unbelievable. But I believe in myself all the time. Of course the support today in Arthur Ashe (Stadium) was crazy. Without you guys, it wouldn't be possible to win this match tonight, so thank you very much for the support.

"I would say 100% of the energy I put in the fifth set was thanks to you. It was unbelievable."

Alcaraz's victory over the 15th-seeded Cilic ensured there will be a new men's Grand Slam champion on Sunday and also kept the Spaniard in contention to become the youngest world number one when the ATP rankings are updated after the final.

The third seed will next meet 21-year-old Italian Sinner, seeded 11th, for a maiden Grand Slam semi-final spot.

Sinner also emerged victorious from a marathon contest against Ilya Ivashka, winning 6-1 5-7 6-2 4-6 6-3 after three hours and 47 minutes.

The 11th-seeded Italian has defeated Alcaraz twice this year - in the fourth round at Wimbledon and in the final of the ATP 250 event in Umag.

"I played a couple of times against him. He's a great player, a really, really tough one," said Alcaraz. "I lost twice in the past two months, so I will have to be ready for this battle against Jannik."