Germany's Alexander Zverev returns the ball to Italy's Jannik Sinner during the US Open tennis tournament men's singles round of 16 match at the USTA Billie Jean King National Tennis Center in New York City, on Sept 4, 2023. (PHOTO / AFP)

NEW YORK - Former US Open runner-up Alexander Zverev beat sixth seed Jannik Sinner 6-4 3-6 6-2 4-6 6-3 late on Monday to move into the quarter-finals of the year's final Grand Slam.

Zverev's win earned him a meeting with defending champion and top seed Carlos Alcaraz, and the 26-year-old German will look to keep his superb run going having hit top gear again after a serious ankle injury cut his 2022 season short.

"I guess I can say I'm back. This is what I live for, this is what I absolutely love to do," Zverev said.

"I would have loved to play a little shorter, that's for sure, but last year when I wasn't able to play, this is what I missed. Playing past 1:30 am in front of a packed crowd on Arthur Ashe Stadium. There's nothing better."

Jannik Sinner, of Italy, serves to Alexander Zverev (not in photograph), of Germany, during the fourth round of the US Open tennis championships, on Sept 4, 2023, in New York. (PHOTO / AP)

Zverev swapped breaks with Sinner early in the contest and was locked in a tight battle until 4-4 before going ahead to take the first set with two aces.

Sinner responded immediately with some heavy hitting in the next set to go up 3-0 and turned up the heat in a steamy centre court to thwart the 2020 finalist's late charge.

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Sinner then struggled with cramp in both legs in the next set. The 22-year-old saved five breakpoints to hold for 2-2 but 12th seed Zverev reeled off the next four games.

World number six Sinner began swinging at everything in the fourth set and Zverev cracked under the onslaught, dropping serve in the ninth game as his opponent forced a decider.

Jannik Sinner, of Italy, returns a shot to Alexander Zverev (not in photograph) of Germany, during the fourth round of the US Open tennis championships, on Sept 4, 2023, in New York. (PHOTO / AP)

But a crucial break in the fifth set gave Zverev the upper hand and he held his nerve to close out the win in four hours and 41 minutes with a powerful serve.

Zverev said he was "completely done" after the fourth set.

"I was extremely tired. He was fitter than me even though he was cramping, he was in better shape. In the fifth set I somehow found it again," he added.

Alexander Zverev, of Germany, serves to Jannik Sinner (not in photograph), of Italy, during the fourth round of the US Open tennis championships, on Sept 4, 2023, in New York. (PHOTO / AP)

"Without the energy, without the crowd this wouldn't have been possible. This is one of the best moments of my career after my comeback, after everything. I'm looking forward to what's next."

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A daunting challenge awaits in the last eight but Zverev said he would be ready.

"I'll give it my absolute best like I always do," Zverev added. "I'll fight until the last moment. Of course, I have to recover after this physical match but I'll be ready."