NEW YORK — When Taylor Fritz, a 26-year-old from California, and Frances Tiafoe, a 26-year-old from Maryland, strode under the Arthur Ashe Stadium lights Friday night for the first U.S. Open semifinal matching two American men in 19 years, the crowd might have been forgiven for not knowing which to support.

There was a burst of clapping right before the initial point, a curtain-raiser befitting the show about to unfold. Once the contest got going, maybe the momentum shifts made it tough to choose between a pair of close pals who’ve known each other since they were playing tournaments for kids younger than 14.

In the end, the roars were for Fritz, who surged with a six-game run against a fading Tiafoe to come out on top 4-6, 7-5, 4-6, 6-4, 6-1 and reach his first Grand Slam final.

“It’s the reason why I do what I do,” Fritz told the fans, his voice cracking during a post-match interview. “It’s the reason why I work so hard.”

The No. 12-seeded Fritz’s seventh victory in eight professional meetings against No. 20 Tiafoe earned a showdown against No. 1 Jannik Sinner for the championship on Sunday.

“He was overwhelming from the baseline so much … and I just tried to tell myself to stay in it and fight,” said Fritz, who was two games from losing in the fourth set. “I told myself that if I didn’t give it absolutely everything I had — to just stick with it and see if his level might drop a little bit — then I was going to regret it for a long time.”

He will be the first U.S. man to appear in a major final since Andy Roddick lost to Roger Federer at Wimbledon in 2009 — and the first in New York since Roddick lost to Federer there in 2006. If he can get past Sinner, Fritz would become the first American man to win any Slam trophy since Roddick got his 21 years ago at the U.S. Open.

“It’s a dream come true. I’m in the final. So I’m going to come out and give everything I can possibly give,” Fritz said. “I can’t wait.”

From 4-all in the fourth set, he seized control. Tiafoe’s mind and legs betrayed him, as he cramped up, at least in part, because of what he said were nerves related to thinking the finish line was near and he was headed to play for the title.

Frances Tiafoe of the U.S. waves to fans after being defeated by Taylor Fritz during the men’s singles semifinals of the U.S. Open tennis championships on Friday in New York.

Seth Wenig/AP

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Seth Wenig/AP

“I felt like my body shut down on me,” said Tiafoe, who fell to 7-14 in five-setters. “I got ahead of myself.”

After his double-fault handed over a break to make it 4-0 in the fifth, more than three hours into the proceedings, Tiafoe chucked his racket. Fritz repaid the favor by double-faulting to end the next game, but broke right back and soon it was over. They met at the net for an embrace.

“Sometimes,” Tiafoe said, “it’s not meant to be.”

Sinner, a 23-year-old from Italy exonerated in a doping case less than three weeks ago, finished off a 7-5, 7-6 (3), 6-2 victory over No. 25 Jack Draper on Friday that featured simultaneous treatment of both competitors by trainers deep in the 1 1/2-hour second set.

“It was a very physical match, as we see,” said Sinner, who won the Australian Open in January. “I just tried to stay there mentally.”

He got his left wrist massaged after falling during a point he managed to win; Draper needed medical attention after vomiting twice in a game with the temperature in the high 70s and the humidity above 60%. During that break in the action, a vacuum was used to clear the ground behind the baseline and finish the cleaning job Draper, a 22-year-old from Britain, tried to do himself by wiping his, um, mess with a towel.

There was none of that sort of drama away from the actual play in Tiafoe vs. Fritz.

“Ultimately,” Tiafoe said, “it was a great night, to have two Americans battling.”

The respective guest boxes seemed to reflect the players’ contrasting personalities. The excitable Tiafoe would mark a key moment by shaking a raised fist or gritting his teeth or wagging his racket or nodding while strutting to the sideline, and his entourage — including coach David Witt, who worked with Venus Williams for many years, as well as Jessica Pegula, the American in the U.S. Open women’s final Saturday against Aryna Sabalenka — stood and got noisy, point after point after point.

The group in the more mild-mannered Fritz’s corner was more selective in its celebrations.

Fritz had never been past the quarterfinals at one of his sport’s four most prestigious events until now, but this journey included wins over a trio of guys with a combined six Slam runner-up showings: Casper Ruud, Alexander Zverev and Matteo Berrettini.

Fritz started well Friday, smacking serves at up to 135 mph, before Tiafoe gathered himself and grabbed five games in a row. In the next set, Fritz was pretty much perfect, winning 24 of 25 service points and going 8 for 8 at the net. Tiafoe regrouped quickly, breaking to begin the third, which turned out to be enough for that set, because he never allowed Fritz so much as one break chance.

Tiafoe appeared to lose steam after losing one particular 31-stroke point — the longest of the match, he called it “definitely an intense rally” — midway through the fourth, then gave away that set’s last game by double-faulting twice and netting a drop shot.

“This one’s going to hurt really, really bad,” Tiafoe said.