Alexander Zverev roars after beating Jannik Sinner ©Getty Images

Alexander Zverev of Germany is set to face Carlos Alcaraz of Spain in the quarter-finals of the US Open after a thrilling five-set win over Jannik Sinner of Italy.

Zverev came out on top 6-4, 3-6, 6-2, 4-6, 6-3 after a gruelling four hours and 41 minutes - the longest match of this year's US Open.

At 4-4 in the first set, the match had reached the one-hour mark - a testament to the fighting spirit showcased by both players.

The third set proved crucial in the end as Sinner cramped up, requiring to briefly stop the game, following which Zverev showed no mercy.

The Italian held on to force a deciding set but could not handle the big serves and forehand winners that sealed the deal for the German.

"I think he was cramping in the third set," Zverev said. 

"In the fourth set, he was physically much better than me. I was struggling a lot.

The biggest upset of the day was when Madison Keys, in picture, knocked out third seed Jessica Pegula ©Getty Images
The biggest upset of the day was when Madison Keys, in picture, knocked out third seed Jessica Pegula ©Getty Images

"And then somehow in the fifth set I picked it up again, I felt much better.

"And then from there on, I got the early break and just served it out from there."

Alcaraz sealed his spot in the last eight with a 6-3, 6-3, 6-4 win over debutant Matteo Arnaldi of Italy.

The defending champion, who dropped only one set in the tournament so far, will be wary of the next game as Zverev holds a 3-2 edge in their rivalry.

Daniil Medvedev, Markéta Vondroušová and Andrey Rublev all secured wins while the biggest upset of the day was when Madison Keys knocked out third seed Jessica Pegula.

The all-American match between Keys and Pegula saw the former start on an attacking note and finish the same way, winning 6-1, 6-3.

Czech star and Wimbledon champion Vondroušová, who will face Keys in the quarters, secured her spot after defeating American Peyton Stearns, 6-7, 6-3, 6-2.

The surprise winner at SW19 is on a 11-match winning streak and will hope to add a second Grand Slam on the trot.

The other men's quarter-final is an all-Russian match up as Medvedev takes on Rublev.

Medvedev reached his fourth US Open quarter-final after defeating Alex de Minaur of Australia 2-6, 6-4, 6-1, 6-2, while Rublev advanced following a 6-3, 3-6, 6-3, 6-4 triumph over Jack Draper of Britain.

New world number one to be Aryna Sabalenka of Belarus eased past Daria Kasatkina of Russia 6-1, 6-3 and will be up against Zheng Qinwen of China next.

It was disappointment again for Tunisian Ons Jabeur, who lost an agonising Wimbledon final, this year.

Zheng beat the fifth seed, who was a finalist here last year, 6-2, 6-4.