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 US Open semi-final 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 and frustrated Tiafoe to come out on top 4-6, 7-5, 4-6, 6-4, 6-1 and reach his first grand slam final.

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

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

“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.”

Fritz 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 US man to appear in a major final since Andy Roddick lost to Roger Federer at Wimbledon in 2009. And if he can get past Sinner, Fritz would become the first US man to win a grand slam trophy since Roddick got his 21 years ago at the US 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.

From 4-all in the fourth set Friday, he seized control as Tiafoe’s strokes and usual confidence betrayed him. After Tiafoe’s double-fault handed over a break to make it 4-0 in the fifth, more than three hours into the proceedings, he 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.

Fritz started well on Friday, smacking serves at up to 135 mph, before Tiafoe gathered himself and grabbed five games in a row. In the next set, Fritz — now 7-1 against Tiafoe — 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 — midway through the fourth, then gave away that set’s last game by double-faulting twice and netting a drop shot.

‘Physical match’

After the first semi-final, Sinner felt: “It was a very physical match...I just tried to stay there mentally.”

Word emerged last month that Sinner failed two drug tests eight days apart in March but was cleared because he said the trace amounts of an anabolic steroid — an ingredient in a treatment for cuts sold over-the-counter in Italy — entered his system unintentionally via a massage from a physio he since has fired.

That whole episode has been a constant topic of conversation as he progressed here.

Sinner appeared to have suffered a wrist injury when he fell over in the second set but rallied to reach his second major final after winning the Australian Open earlier this year.

Little went right for Draper, who vomited three times in the second set as he struggled with the heat and humidity that also left his shirt soaked in sweat. Ten double faults also were a deterrent to the 22-year-old’s bid to reach a grand slam final for the first time.

During the match, one physio came to massage the Italian’s wrist while another came to help the ailing Draper during the changeover.

Draper requested another set of fresh shoes from his locker and battled on, vomiting for a third time in the 12th game before Sinner sprinted through the tie-break. Sinner closed out the match with a backhand winner on match point.

Doubles crown

Latvian Jelena Ostapenko and Ukraine’s Lyudmyla Kichenok won the women’s doubles title, beating Chinese-French pair Zhang Shuai and Kristina Mladenovic 6-4, 6-3 after coming up short in the Australian Open final.

Men’s final: Sunday 11.30 pm. Live on Sony Sports Network.