Alexander Zverev defeated Jannik Sinner in a late-night five-set US Open marathon to line up a quarter-final with defending champion Carlos Alcaraz.

German 12th seed Zverev outlasted Sinner 6-4, 3-6, 6-2, 4-6, 6-3 in a gruelling 4hr 41min match which finished at 1.39am local time Tuesday on a muggy night in New York.

Zverev claimed his second win over the sixth-seeded Italian in three years at Flushing Meadows and advanced to a third quarter-final here after the longest US Open match of his career.

"I guess I can say I'm back. This is what I live for, this is what I absolutely love to do," Zverev said in an on-court interview.

"Last year when I wasn't able to play this is exactly the kind of moments I missed."

Zverev, the 2020 US Open runner-up, missed last year's tournament after suffering a horror ankle injury at Roland Garros.

"I actually don't know how I managed that. I was completely done in the fourth set. I was extremely tired. In the fifth set I somehow found it again," said Zverev.

"This is one of the best moments of my career, after my comeback and everything. I'm looking forward to what comes next."

The 26-year-old Zverev has a 3-2 record against world number one Alcaraz, winning their only Grand Slam meeting in the 2022 French Open quarter-finals, and is regaining his best form.

"I'm here to play. That's what I love doing. I don't know how it'll end up but I'll give it my absolute best like I always do," he said.

"I'll fight till the last moment. I'll be ready."