A decade or so ago, back when Tommy Paul, Taylor Fritz and Frances Tiafoe were rowdy teenagers raising hell at the United States Tennis Association dormitories in Florida, they dreamed that days like Sunday at the U.S. Open would eventually come.

Coco Gauff and Ben Shelton were barely 10 years old back then, still figuring out how large a role tennis was going to play in their childhoods, though it was a safe bet it would be pretty large.

Flash forward to Sunday at the U.S. Open, and those five players were at the center of what figured to be a daylong American tennis festival in the fourth round, a part of the tournament when, for so long, especially on the men’s side, players from Europe have filled the starring roles. Not on Sunday, when the year’s final Grand Slam tournament got down to serious business and the round of 16.

The schedule featured wall-to-wall red, white and blue; Black and white and mixed race players; players from wealthy families (Fritz), from more humble means (Shelton, Gauff, Paul), and one (Tiafoe) who started with almost nothing; some players with years of tour experience and one so raw (Shelton) that he needed to get a passport last year so he could leave the United States for the first time to play in the Australian Open.

“We always believed this would happen,” said Martin Blackman, the general manager for player development at the U.S.T.A., who has known all five players since their early years. “But you never know when.”

When Serena Williams, a majestic and groundbreaking figure in sports and culture for more than two decades, retired from pro tennis at this tournament last year, she left big questions about who might begin to fill the massive void she was leaving, especially in American tennis. Some pretty good hints arrived within days. Gauff and Tiafoe — charismatic figures with bright eyes and big smiles who play with equal parts heart, skill and athleticism — blazed into the deep end of the 2022 tournament, the quarterfinals for Gauff and the semifinals for Tiafoe.

That was last year, though, and there was no guarantee that they or any of their compatriots would reproduce the magic of some of those days. Sunday represented a decent midpoint indicator.

Looking at the draw in the middle of last week, Fritz’s eyes drifted to the quarter just above him, where Shelton, Paul and Tiafoe were crowded together. Some big names were out, and his people were still very much alive. Immediately he thought, “One of them is going to be in the semis,” and that was pretty cool.

Paul and Shelton got the action rolling at noon Sunday in the opening match at Arthur Ashe Stadium. The stands were filling up more with every changeover, getting louder each time Shelton’s booming serve put up big numbers on the radar gun.

Two adrenaline-fueled blasts clocked in at 149 miles per hour as he built a commanding two-set lead before Paul came alive with the crowd rallying behind him. The stadium was near its capacity of 23,000 by the time his last forehand sailed long. It wasn’t the outcome Paul wanted, but the match had its moments.

Early on, he looked up at the video board and saw that he and his buddies were on the list of Americans left in the tournament. He let that sink in, those names from the dormitory hall, names that were there in the late rounds of the junior national tournaments in his teenage years.

“We grew up all together,” Paul said shortly after the loss. “Kind of cool.”

Every Grand Slam tournament crowd throws its weight behind its home-country players. At the Australian Open, the “Aussie, Aussie, Aussie, Oy, Oy Oy!” chant is a constant refrain. French crowds break out in spontaneous renditions of “La Marseillaise.” At Wimbledon, Britons will pack a field court to urge on a junior player they have never heard of with the same vigor they offer Andy Murray.

The U.S. Open crowd, by reputation the rowdiest and most indecorous of them all, does its boisterous best to get its own over the line.

Shelton, 20, hugged Paul at the net wanting to hear just what full-throated screams from the biggest crowd he had ever played before might sound like. Hard to blame him on that front.

“Amazing atmosphere, felt the love all day,” he said on the court moments later.

And it stayed that way as Gauff played against Caroline Wozniacki, a former world No. 1. Wozniacki is on the comeback trail after having two children and has long been a crowd favorite in New York.

That said, she had never played Gauff on a day that felt like a flashback to a couple generations ago, back to the eras when American men and women always held the promise of becoming the class of the sport and were among its biggest stars. This was part tennis match, part revival meeting, with more screams of “Go Coco!” than anyone could count in a building that Gauff, who is just 19, figures to be making her home for the next decade.

A slight complication, a welcome one for the hometown crowd, arose as 4 p.m. approached when Tiafoe strutted into Louis Armstrong Stadium to play Rinky Hijikata of Australia just as Gauff was finding her groove. Like a parent facing a choice between children, Blackman needed a plan.

“First set with Coco, then over to Frances,” he said as he rushed through a hallway underneath the stadium.

Slight complication for Gauff, too, in the form of a late-second and early third-set wobble that had her hitting backhand after backhand into the middle of the net. Wozniacki surged into the lead, breaking Gauff’s serve in the first game of the third set. But Gauff and her 20,000 friends weren’t about to let that last for long, not on this day. With a slew of “Come ons!” and teeth clenches she reeled off the final six games, bulldozing her way back into the quarterfinals.

“Had some chants going, which was really nice.” Gauff said later. “The crowd doesn’t really compare to any of the other Slams.”

She won two of the three U.S. Open tuneup tournaments and, despite dropping sets in three of her first four singles matches, is brimming with confidence.

“I’ve been in this position before,” said Gauff, a French Open finalist last year. “I can go even further.”

Meanwhile, over on Armstrong, Tiafoe was cruising.

If Ashe is American tennis’s grand cathedral, Armstrong is its party space, a 10,000-seat concrete box with an upper level of seating that seems to hang almost directly above the court and a retractable roof that keeps sound echoing up and down and all around even when open. And no one these days, other than Carlos Alcaraz, knows how to throw a party like Tiafoe, 25, who broke into the top 10 of the rankings for the first time earlier this year.

The drunker and more spirited the fans the better as far as he is concerned. He pumps his fists, shakes his racket, and even throws out the occasional tongue wag after those curling forehands and jumping two-handed backhands, to make it just how he likes it, with as many hollers of “Go Big Foe!” as he can wring from them. It’s how he has long believed American tennis should be, and part of the reason he is Paul’s favorite player to watch in the sport.

Up next for Tiafoe is Shelton, and he wouldn’t have it any other way.

“He’s going to come after me, and I’m going to come after him,” he said. “I plan on being in the semi.”

Then it was Fritz’s turn, filling the early evening slot on Armstrong, and taking the court shortly after Tiafoe left it, against Dominic Stricker, 21, of Switzerland, one of the surprises of the tournament. Stricker had to win three matches in the qualifying tournament to get into the main draw and he upset Stefanos Tsitsipas, a two-time Grand Slam singles finalist, in the second round. He had already played 22 sets of tennis in New York, including two five-setters, before he hit his first ball against Fritz.

Much of the Tiafoe crowd filed down the stairs into the main plaza of Billie Jean King National Tennis Center. Waiting at the bottom were thousands more ready to take their place, Honey Deuces, Aperol spritzes, beers, poke bowls and fries in hand.

Three American headliners had already moved on. Madison Keys and Jessica Pegula were set to play each other in the fourth round Monday, and Peyton Stearns, out of Ohio and the University of Texas, was set to take on Marketa Vondrousova, this year’s Wimbledon champion. This home-country party was rolling on.