American Taylor Fritz reacts after his victory over Frances Tiafoe during the semi-finals of the US Open tennis championship on Friday in New York.

Seth Wenig/AP

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Seth Wenig/AP

NEW YORK — When Taylor Fritz, a 26-year-old from California, and Frances Tiafoe, a 26-year-old from Maryland, appeared under the lights of Arthur Ashe Stadium on Friday night for the first U.S. Open semifinal to face two American men in 19 years, the crowd could have been forgiven for not knowing who to cheer for.

There was a clap just before the starting line, an opening act befitting the show about to unfold. Once the match got underway, the shifts in momentum may have made it difficult to choose between a pair of good friends who had known each other since playing in under-14 tournaments.

In the end, there was cheering for Fritz, who won a series of six matches against the weaker Tiafoe, eventually winning 4-6, 7-5, 4-6, 6-4, 6-1. He thus reached his first Grand Slam final.

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

Twelfth-seeded Fritz’s seventh victory in eight professional encounters against twentieth-seeded Tiafoe earned him a championship showdown with top-seeded Jannik Sinner on Sunday.

“He was so overwhelming from the baseline … and I just tried to tell myself to keep fighting,” said Fritz, who was two games away from losing the fourth set. “I told myself that if I didn’t give absolutely everything I had — just keep going and see if maybe his level would drop a little bit — I would regret it for a long time.”

He will be the first American 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 beat Sinner, Fritz would become the first American to win a Grand Slam trophy since Roddick won it at the US Open 21 years ago.

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

From 4-all in the fourth set he took control. Tiafoe’s mind and legs betrayed him as he started to cramp, at least partly due to what he said were nerves, as he thought the finish line was near and he was on his way to playing for the title.

US’s Frances Tiafoe waves to fans after being defeated by Taylor Fritz in the semi-finals of the US Open tennis championship in New York on Friday.

Seth Wenig/AP

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Seth Wenig/AP

“I felt like my body was giving out,” said Tiafoe, who trailed 7-14 in five-setters. “I was getting ahead of myself.”

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

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

Sinner, a 23-year-old Italian who was acquitted in a doping case less than three weeks ago, claimed a 7-5, 7-6 (3), 6-2 victory over No. 25 Jack Draper on Friday. Both players were treated simultaneously by trainers during the 1 1/2-hour second set.

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

His left wrist was being massaged after he fell during a winning point; Draper required medical attention after vomiting twice in a game that featured temperatures in the 70s and humidity in excess of 60 percent. During that lull in the action, a vacuum cleaner was used to clear the ground behind the baseline, finishing the cleanup job that Draper, a 22-year-old from Britain, was attempting to do himself by wiping up his, er, mess with a towel.

There was no drama outside of the actual play Tiafoe vs. Fritz.

“Ultimately,” Tiafoe said, “it was a great night, to see two Americans fight each other.”

The respective guest boxes seemed to reflect the players’ contrasting personalities. The excitable Tiafoe would mark a key moment with a raised fist or grit his teeth or a sway or a nod of his racket as he strutted toward the sidelines, and his entourage — including coach David Witt, who worked with Venus Williams for years, and Jessica Pegula, the American in Saturday’s US Open women’s final against Aryna Sabalenka — would rise and become boisterous, point after point after point.

The group in Fritz’s corner, who took it a bit easier, were more selective in their celebrations.

Until now, Fritz had never advanced beyond the quarterfinals of any of the four most prestigious events in his sport, but the journey did include victories over three guys who had finished runner-up at a Grand Slam six times between them: Casper Ruud, Alexander Zverev and Matteo Berrettini.

Fritz started off strong on Friday, hitting serves up to 135 mph, before Tiafoe regrouped and took five games in a row. In the next set, Fritz was nearly perfect, winning 24 of 25 service points and going 8 for 8 at the net. Tiafoe quickly regrouped and broke to start the third, which proved to be enough for that set, as he gave Fritz no chance to break.

Tiafoe appeared to lose steam after losing a 31-shot point midway through the fourth set — the longest of the match, he called it “an absolutely intense rally” — and then gave away the final game of that set by double-faulting twice and hitting a drop shot.

“This is going to hurt really bad,” Tiafoe said.