Former champion Carlos Alcaraz's Grand Slam winning streak came to a screeching halt at the U.S. Open on Thursday as the Spanish third seed was thumped 6-1 7-5 6-4 by unseeded Dutchman Botic van de Zandschulp in the second round.

An off-colour Alcaraz never recovered from a shaky start and made unforced errors throughout at a stunned Arthur Ashe Stadium as the inspired Van de Zandschulp snapped the French Open and Wimbledon champion's 15-match winning run at the majors.

It was the Paris Olympics silver medallist's earliest exit at a Grand Slam since Wimbledon in 2021.

"I don't know what to say right now. First of all, I think he played great," Alcaraz said.

"He didn't make a lot of mistakes that I thought he was going to do so I was confused a little bit.

"I didn't know how to manage that."

The 21-year-old was on the back foot early on as Van de Zandschulp claimed the opening three games with some resolute defending from the back of the court, and the Dutchman broke again in the sixth game en route to taking the first set.

Alcaraz produced a stunning forehand pass to hold serve in the first game of the next set but the four-times major winner's struggles to close out points resurfaced as Van de Zandschulp broke for a 2-1 lead.

A tactical tweak to return serve from deeper helped Alcaraz break back immediately but the 2022 champion gifted his opponent another break with a double fault and Van de Zandschulp went on to comfortably double his advantage in the match.

After briefly leaving the court before the third set, Alcaraz found himself in deeper trouble after a wayward forehand handed Van de Zandschulp a break.

He hit back immediately and found his smile again, but world number 74 Van de Zandschulp got his nose in front and completed a stunning upset on serve.

"I didn't feel well hitting the ball," Alcaraz added. "I think I made a lot of mistakes and when I wanted to come back ... it was too late."

Former New York quarter-finalist Van de Zandschulp, hampered by injuries to his left foot in the last two years, was lost for words after the biggest victory of his career.

"It's been an incredible evening. First time for me having a night session on Arthur Ashe. The crowd was amazing. Thank you for that. Unbelievable night," said Van de Zandschulp.

"I think from point one here today I believed (I had) a chance. I had some nerves but if you want to beat one of these guys you have to be unbelievably calm and keep your head there."

Van de Zandschulp will next face Britain's Jack Draper.