Carlos Alcaraz of Spain, left, defeats Li Tu of Australia at USTA Billie Jean King National Tennis Center. | Robert Deutsch-USA TODAY Sports

Carlos Alcaraz is only 21, already has four Grand Slam titles and is going for No. 5 at the U.S. Open. He also has opponents raving over his muscles.

After being defeated by the third-seeded Alcaraz in four sets Tuesday, Li Tu of Australia told ESPN that he was impressed by the Spaniard during warmups.

“I’m not gonna lie, he came out and he’s in his jacket and I was like, ‘OK,’” Tu said. “Then we go to the coin toss and he took his jacket off. And I was like, ‘Dude, this guy’s a specimen.’ From the coin toss, I was like, ‘OK, this guy’s a beast.’ Like I just couldn’t stop looking at his shoulder definition and the veins. And the forehand says it. He’s got some serious, serious weaponry. So, hat’s off to him.”

Tu was not the only men’s player to notice how buff Alcaraz is. This is what Nick Kyrgios told Alcaraz:

Kyrgios. “Your arms look sensational in that outfit.”

Alcaraz. “I love to play in sleeveless shirts.”

Kyrgios. “I know you do.”

Training hard and buffing out is a deliberate strategy for Alcaraz to build his strength and stamina. His personal trainer said last year that the hardest part was getting Alcaraz to consume 4,500 calories a day.

“He ate very little. He had the habits of a normal boy, not an athlete,” the trainer said. “That’s what cost us the most with him.”

Whatever Alcaraz is doing is obviously working and showing off the fruits of his hard work is a nice perk that gets people’s attention, even that of his opponents.