NEW YORK — Two days after making his auspicious introduction to tennis fans, Carlos Alcaraz took the court in front of a capacity crowd of more than 8,000 in Grandstand stadium for his fourth-round match at the US Open on Sunday. Even with the looming threat of rain, there was an audible buzz about the 18-year-old sensation.

“Vamos, Carlos!” yelled one fan.

“I heard he was the next Rafa,” another said to his friends.

Three-and-a-half hours and five sets later, Alcaraz had won yet again — coming back from a set down entering the fourth against Peter Gojowczyk — to become the youngest man to advance to the quarterfinals at the tournament in the Open Era. It was just the third five-set match of Alcaraz’s career and his second one in less than 48 hours. Those in attendance cheered raucously for his every point and frequently chanted for him during breaks.

Alcaraz became the toast of New York on Friday following his upset victory over No. 3 seed Stefanos Tsitsipas on Arthur Ashe Stadium, but tennis insiders have long known about Alcaraz’s powerful game and limitless potential. To them, reaching the second week of a major isn’t a surprise.

It just wasn’t supposed to be this early.

“I didn’t expect to play quarterfinals here,” Alcaraz said Sunday.

Then again, he is no stranger to the “youngest ever” or “youngest since” distinction, as his rapidly growing résumé is full of such accolades. Next, Alcaraz will take on No. 12 seed Felix Auger-Aliassime on Tuesday with a chance to reach the semifinals — and show more people what Tsitsipas discovered for himself.

“He can be a contender for Grand Slam titles,” Tsitsipas said after their match. “He has the game to be there.”

Alcaraz grew up idolizing fellow Spanish countryman Rafael Nadal, and his celebratory on-court mannerisms often show an uncanny resemblance. But it was teaming up with another of Spain’s great tennis stars that helped propel Alcaraz to the sport’s highest of stages.

In 2018, he began working with Juan Carlos Ferrero, the former world No. 1 and 2003 French Open champion, who remains his coach. Alcaraz turned pro the same year. Ferrero immediately recognized his young protégé’s talent but knew he would have to find a way to rein in his power and maximize his aggression.

“Since I met him when he was 14, 15, I knew of his potential, about his level,” Ferrero said on Saturday. “But definitely to be that aggressive, you have to control yourself and be able to manage all the shots that you’ve got about the aggressivity that he has. That it’s not easy, because sometimes, you know, comes to your mind a lot of ideas to hit the ball, and you have to try to not put all those things together to not play as a plan.

“So, Carlos start to manage all these things on the court. Off court, he’s still 18 years old, and he needs to get more mature, to control his emotions in there and to control when he has to go with 100 percent of his potential or when he has to use 80 percent or sometimes play with a lot more spin or more flat. So, he’s on the way to order all these kind of things, but I think he’s in a good way to do it.”

Alcaraz can often be seen looking — or screaming or shaking his fists in jubilation — in Ferrero’s direction while he is on the court. And as Ferrero has helped take Alcaraz’s career from the sport’s lower levels to the present heights, the need for his validation perhaps makes sense.

Alcaraz won his first ITF Futures title as a 16-year-old in July 2019 then opened 2020 with back-to-back titles from the same tour. His early-season success got him a wild card into the Rio Open in February.

In his first ATP tour-level match at the event, he defeated former world No. 17 Albert Ramos-Vinolas in a hard-fought battle lasting 3 hours, 36 minutes. Alcaraz became the youngest player to win a match on tour since 2013 and the first to do so of those born in 2003. Even with the suspension of the tour just weeks later due to the pandemic, Alcaraz found a way to raise his ranking by 350 spots by the end of the year — jumping from No. 491 to No. 136.

He won three Challenger titles following the restart and was named the ATP’s Newcomer of the Year.

Alcaraz faced Nadal in the second round of the Madrid Open in May 2021. Alcaraz lost the match 6-1, 6-2, but impressed the 20-time major champion in their first meeting.

“When you make a salad and you are putting ingredients inside the salad, he has plenty of ingredients to become a great player,” Nadal said. “That’s the main thing. Then, of course, nothing is easy. You’re gonna have big opponents in front. I mean, nothing is easy in this life.

“Be[ing] one of the best players in the world and fight for the most important titles is something very difficult, but I really believe that he’s one of the guys that can do it.”

Alcaraz won his first ATP title at the Croatia Open in July with a 6-2, 6-2 decision over former top-10 player Richard Gasquet in the final. With the victory, he became the youngest titlist on tour since Kei Nishikori in 2008.

The teen told ESPN in December his goals for the new season were to make the main draw in all of the majors and crack the top 50. He has done both of those — and then some.

He reached the second round at the Australian Open and Wimbledon and made the round of 32 at Roland Garros. Currently ranked No. 55, his win over Tsitsipas pushed Alcaraz to a projected ranking of No. 50, and he is expected to rise to No. 38 after Sunday’s victory.

A win in the quarterfinals would put Alcaraz in the top 30.

While Brad Gilbert, the former world No. 4 and an ESPN analyst, was very familiar with Alcaraz’s game entering the US Open, even he was shocked by what he saw in the third-round upset over Tsitsipas.

“I had no idea he could hit that big,” Gilbert said. “I think what surprised me most was how he was crushing the ball off of both sides during the first four games of that match. He just absolutely crushes the ball.

“I didn’t think he could serve above 126, 127 [mph], but he was popping 134. He was hitting many forehands over 100 miles an hour. I had no idea he could do that. It was great to witness live.”

Alcaraz was in the unfamiliar position of being the favorite on Sunday against Gojowczyk, the 32-year-old qualifier from Germany. Alcaraz said it didn’t change the way he prepared for the match, but he struggled at times — he served for the first set at 5-4 but was broken and then lost four consecutive games — and he was wildly inconsistent throughout. But when it mattered most, in the final set, he was dominant. He didn’t lose a game in the decider and took the match 5-7, 6-1, 5-7, 6-2, 6-0.

While he had thrown his racket and fallen to the ground — where he lay outstretched with his hands over his face — after defeating Tsitsipas, this time Alcaraz was more poised, more prepared, for the moment. He bent down slightly and joyously shook his arms, and he later blew kisses to the crowd and posed for selfies with fans. He said afterward he was just happy to have advanced.

“I’m super excited to be in my first second week in [a] Grand Slam,” Alcaraz said. “It’s amazing for me. It’s a dream come true.”

On Tuesday, he’ll return to the underdog role, albeit much less so now than in the third round for the clash with Auger-Aliassime. Alcaraz has never competed against the 21-year-old Canadian player, who also is seeking his first major semifinal appearance.

Alcaraz was the second of two teenagers to advance on Sunday to the final eight, after Leylah Fernandez, 19, defeated Angelique Kerber in the women’s draw, with Emma Raducanu, 18, playing for a spot on Monday. With their success — and that of a number of other young players such as Auger-Aliassime and previous major champions Bianca Andreescu, 21, and Iga Swiatek, 20, who both remain in the women’s draw — it seems as if tennis’ future has arrived.

And no matter what happens in the quarterfinals or beyond, Alcaraz has made a believer out of many during his remarkable run at the US Open.

“He’s coming in like a freight train,” Gilbert said. “I don’t want to say, ‘Oh, he’s going to win this or that,’ but I will say this: If he was a stock, I would put a ‘buy’ rating on him.

“The great thing is he’s got a really good coach, and it’s all about getting better. Keep improving his game and his movement, keep getting stronger. There’s no doubt he has the potential to be a top-5 player and could obviously be even better.”