Germany's Alexander Zverev came through the longest match of this year's US Open to beat sixth seed Jannik Sinner and reach the quarter-finals.

The 2020 runner-up missed last year's hard-court Grand Slam event because of a serious ankle injury.

"This is one of the best moments of my career after the injury and the comeback," said Zverev.

Earlier, third seed Daniil Medvedev beat Australia's Alex de Minaur to set up a quarter-final with fellow Russian Andrey Rublev.

Eighth seed Rublev knocked out Britain's Jack Draper earlier on Monday before his close friend Medvedev fought back to win 2-6 6-4 6-1 6-2 against 13th seed De Minaur.

Zverev, 26, became the final man to book his place in the last eight, coming through a gripping contest against Italy's Sinner, which lasted four hours and 41 minutes.

"I guess I can say I'm back. This is what I live for," said Zverev.

"I would have loved to play a little bit shorter but last year when I wasn't able to play, this is what I missed."

Earlier on a humid night in New York, 2021 champion Medvedev came through in what he called "brutal" conditions.

"Some people are better with the heat, some people worse. I feel like I'm not the best one in this case," said Medvedev.

"At the same time, I feel like I fight many times through it quite well, so that's kind of what happened.

"I managed to raise my level. It was an amazing last three sets, so I'm really happy. I hope I can play the same way going further."

Alcaraz ends Arnaldi's dream run

Spain's Alcaraz continued his defence of the title by registering a 6-3 6-3 6-4 win over world number 61 Arnaldi.

Alcaraz was formidable on Arthur Ashe Stadium, hitting 31 winners as he booked his spot in the quarter-finals with win in one hour and 59 minutes.

The 20-year-old, who beat British number two Dan Evans in the third round, broke midway through the first set and served out the opener to love.

Arnaldi, playing in his first Grand Slam fourth round, was then broken in the first game of the second set as Alcaraz continued to dominate.

The US Open debutant, 22, has had a dream run this fortnight, beating British number one Cameron Norrie last time out, but a shock against two-time major winner Alcaraz always looked a step too far.

The world number 61 did get a break of his own to take a 2-1 lead in the third set but was immediately broken back and, trailing 5-4, was unable to hold serve to stay in the match.

Alcaraz has now won 15 of his past 16 matches at the US Open and is a strong favourite to defend his title.

"When I won Wimbledon I said I'd fallen in love with the grass," said Alcaraz, who won the second major title of his career at the All England Club in July.

"I'm really comfortable with the three surfaces but right now the hard court is my favourite one."