NEW YORK (AP) — Everyone kept waiting for Carlos Alcaraz to turn things around at the U.S. Open.

Alcaraz figured it would happen at some point. So did his opponent. And surely the Arthur Ashe Stadium crowd and folks tuning in on TV did, too. This is, after all, Carlos Alcaraz we’re talking about — the 21-year-old wunderkind with four Grand Slam titles already, including one at Flushing Meadows as a teen.

A guy at the top of the game right now. A guy expected to accept the mantel from the Big Three of Novak Djokovic, Rafael Nadal and Roger Federer. A guy who entered the U.S. Open as the favorite and went into the second round in New York on a 15-match winning streak at the majors, with championships at the French Open in June and Wimbledon in July, plus a silver medal at the Paris Olympics in early August.

The best version of Alcaraz never materialized on Thursday night in Arthur Ashe Stadium against 74th-ranked Botic van de Zandschulp, who wound up winning 6-1, 7-5, 6-4, a result as stunning for who won as for how easily he did.

Afterward, the No. 3-ranked Alcaraz sounded like someone a little worried about what it might mean.

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“Instead of taking steps forward, I’ve taken steps back mentally. I can’t understand the reason why,” he said during the Spanish portion of his post-match news conference. “I have to check what’s going on with me.”

What happened to Carlos Alcaraz at the U.S. Open?

It wasn’t just that Alcaraz sounded defeated.

It was also that he sounded bewildered.

“I couldn’t see the ball well. ... I couldn’t hit it properly. It’s quite a weird sensation,” Alcaraz said. “I’m not well mentally, not strong. I don’t know how to manage the difficult moments, and that’s a problem for me.”

Who is Botic van de Zandschulp?

Across the net was van de Zandschulp, a 28-year-old from the Netherlands who seriously contemplated retirement a few months ago and came to the U.S. Open with a record of 11-18 this season and without back-to-back victories at any tour-level tournament.

He only once has made it as far as the quarterfinals at any Grand Slam tournament, getting to that stage at Flushing Meadows three years ago.

So van de Zandschulp was pretty sure the one-sided nature of Thursday’s match was going to shift.

“Even in the third, you’re thinking, like, ‘He’s going to come up with something special,’” van de Zandschulp said. “I actually was thinking that the whole match.”

But Alcaraz just was unable to get going.

Why did Carlos Alcaraz struggle at the U.S. Open?

He couldn’t really explain why he never turned things around or why he failed to find something that would work.

“Today I was playing against the opponent, and I was playing against myself, in my mind,” Alcaraz said. “A lot of emotions that I couldn’t control.”

When a reporter offered one possible explanation — exhaustion after what’s been a busy stretch — Alcaraz did acknowledge a tennis schedule he called “so tight” could have been too draining.

He went from the clay of Roland Garros to the grass of the All England Club to the clay of the Summer Games and then to the hard courts of North America.

“Probably, I came here with not as much energy as I thought that I was going to (have),” Alcaraz said. “But, I mean, I don’t want to put that as excuse.”

What comes next for Carlos Alcaraz?

Maybe the devastating loss to Novak Djokovic in the Olympic final that left Alcaraz in tears was hard to process properly. In the one hard-court match he played before the U.S. Open — a defeat against Gael Monfils at the Cincinnati Open — Alcaraz lost his cool, repeatedly smashing his racket on the court, a reaction he later apologized for.

Now he’s dropped three of his past four contests and needs to come up with a way to move past this stretch and be ready for the next Grand Slam tournament, the Australian Open in January.

Then again, maybe Alcaraz shouldn’t be too hard on himself. After all, there must be a reason only two men in the past 55 years managed to win the championships in Paris, London and New York in a single season: Rod Laver in 1969 (when he completed a calendar-year Grand Slam) and Rafael Nadal in 2010.

“I have to think about it,” Alcaraz said. “I have to learn (from) it ... if I want to improve.”

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AP Sports Writer Eric Núñez contributed to this report.

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Howard Fendrich has been the AP’s tennis writer since 2002. Find his stories here: https://apnews.com/author/howard-fendrich

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AP tennis: https://apnews.com/hub/tennis