Carlos Alcaraz and Jannik Sinner are the latest evolution on the men’s side. Building off what the Big 3 did over the last two decades, the young Spaniard and Italian have taken the sport another step forward. They hit bigger, run faster, slide harder, and belt winners from farther afield than anyone before them. It’s way too early to put them in any kind of GOAT debate, but they can already hold their own when we talk about which players have had the highest ceilings.

For all of that progress, though, Alcaraz’s 6-2, 3-6, 6-1, 6-4 win in their US Open final came down to an old-fashioned element of the game, one that has been making the difference between winners and losers for centuries: The serve.

Alcaraz’s was good. Sinner’s wasn’t.

Alcaraz hit 10 aces and committed zero double faults. He made 61 percent of first serves and won 83 percent of those points. And he won 57 percent of points on his second serve.

The gap grew as the match progressed. Alcaraz regularly reached the 130s, and by the concluding fourth set, he was in a fearsome serving rhythm. At 3-2, he closed out a hold with two straight 132-m.p.h. aces. At 4-3, he had an ace and a service winner in a speedy love hold. At 5-4, serving for the title. he hit one 134 (Sinner returned it), and finished the match with a 131 service winner.

For a guy who was trying to win a Slam title, Alcaraz had an easy swagger, like someone who knows he has an unstoppable go-to shot in his back pocket. At 5-4, he went up 40-15, double match point, but lost both of those points. On the second one, Sinner rocketed a backhand that brought the Ashe crowd to its feet. Was it Sinner’s turn to save match points in a Slam final, the way Alcaraz had at Roland Garros?

Rather than tighten up at that moment, Alcaraz went back to work. He threw down a big serve, followed it with a powerful backhand, finished the point with a smash, and walked back to the baseline grinning.

“This change, it comes from the Australian Open,” Alcaraz’s coach, Juan Carlos Ferrero, said on Sunday. “I think this past December we decide to change a little bit his movement of the serve. During all Cincinnati and also during all US Open, I think the serve is one of the keys to win the tournaments.”

Which takes me back to the evolution question. Even if this match was decided in a traditional way, we did see a step forward on Sunday, and Alcaraz took it. Whenever he plays Sinner, he goes back to the tape and assesses what he did well, what he did poorly, and what he’ll try to do differently the next time. It helps, he says, that he’s a fan of Sinner’s game.

“First of all, because I love watching him play,” Alcaraz says of why he watches so much of Sinner. “I think it is unbelievable what he’s doing. Secondly, it’s because I love to study him, how he plays, how he feel on the tournaments just to [see] if I played him in that tournament, just to have feedback how he’s been playing.”

After his defeat at Wimbledon, Alcaraz said of Sinner, “From the baseline I was feeling he was better than me, and I couldn’t do anything about it,” and that “He was returning really well the second serve; thanks to that, he was in position to attack the second ball every time.”

“I studied the Wimbledon final…We watch it. We note everything, and we work on it…I just thought about the specific things I want to improve if I want to beat Jannik.”

Alcaraz understood that he had been beaten by a more aggressive player, and he decided he wouldn’t lose that way again. In the three sets that he won on Sunday, he was the one who hit from farther up in the court, who followed the ball forward—even on his drop shots—and who got ahead in the rallies. It’s not a coincidence that the one set Alcaraz lost was the one where he was content to rally, which allowed Sinner to start taking the first strike.

Alcaraz was 20 of 27 at net, and finished with twice as many winners (42 to 21) and fewer unforced errors (24 to 28) than Sinner.

Alcaraz isn’t the only one in this rivalry who can evolve, of course. By the time he made it to the interview room, Sinner was already thinking of what he needed to do in response.

“Now it’s going to be on me if I want to make changes or not, you know? That’s definitely we are going to work on that,” Sinner said. “During this tournament, you know, I didn’t make one serve-volley, didn’t use a lot of drop shots.”

“I’m going to aim to—maybe even losing some matches from now on— but trying to do some changes, trying to be a bit more unpredictable as a player.”

Back and forth, hopefully, they go. For today, though, it’s Alcaraz who is back in the lead, and back at No. 1 in the world for the first time in two years.

“I feel like this is the best tournament,” Alcaraz said. “Since the first rounds to the end of the tournament, the best tournament so far that I have ever played.”

“It was one of the first goals that I had during the season, to try to recover No. 1 as soon as possible. For me, [to] achieve that once again, it is, as I said, it’s a dream.”