Carlos Alcaraz's 15-match winning streak in Grand Slam tournaments came to a stunning end on Thursday night with a straight-sets loss to Botic van de Zandschulp in the second round of the U.S. Open.

After the defeat, Alcaraz told reporters he was "playing against myself" trying to get out of his own head during the match:

"It was a fight against myself, in my mind, during the match. In tennis, you are playing against someone that wants the same as you—to win the match—and you have to be as ... calm as you can, just to think better in the match and try to do good things. Today I was playing against the opponent, and I was playing against myself, in my mind. A lot of emotions that I couldn't control."

There were indications in the first round that Alcaraz wasn't at the top of his game. He dropped the second set to qualifier Li Tu on Tuesday by committing 18 unforced errors.

Alcaraz ultimately defeated Tu in four sets, but it was a much more grueling match than anyone anticipated.

Things never came together for Alcaraz on Thursday night. He lost the opening set in just seven games and later double-faulted to give van de Zandschulp a 6-5 lead in the second set.

On Saturday, Alcaraz had to cut a training session short after twisting his right ankle. He said afterward it was "okay" and the decision to stop practicing was precautionary.

Even though Alcaraz didn't blame any injuries for the defeat, he did talk about feeling drained because of the schedule he's played recently.

"Probably, I came here with not as much energy as I thought that I was going to [have]," he said. "But I mean, I don't want to put that as excuse."

Since the French Open started on May 20, Alcaraz has played 28 matches in singles and doubles competition. This stretch includes winning Wimbledon and reaching the gold-medal match at the Olympics.

Alcaraz came into the U.S. Open following a loss to Gael Monfils in his first match at the Cincinnati Open on Aug. 15.

Thursday's loss marks the first time Alcaraz has lost at a Grand Slam tournament since the quarterfinals of the Australian Open in January. This marks the first time he has lost before the quarterfinal of any Grand Slam event since Wimbledon in 2022.