Martin Blackman doesn’t know what the Arthur Ashe Stadium atmosphere will be like Friday — and the USTA general manager of player development said he would be faking it if he even tried to predict — for a semifinal matchup 15 years in the making.

But he’ll still scan the crowd for an open seat, venture toward the fans in that area, sit on his hands and then watch.

Maybe, Blackman said, the Frances Tiafoe-Taylor Fritz showdown in an all-American semifinal at the U.S. Open will mirror a vintage Pete Sampras-Andre Agassi one from the 1990s, where the crowd split its loyalty yet still roared when the other hit a thrilling shot.

He hopes that’s the case.

“The New York crowd’s gonna pick their sides,” Blackman said Wednesday night.

But either way, he said, “it’s a win-win atmosphere,” because at the end of Friday’s match, either Tiafoe or Fritz will become the first American man to make a Grand Slam final since 2009.

Then, the winner will have a chance to become the first to win a major title since Andy Roddick’s victory in Queens 21 years ago.

Roddick remains the link to the other end of the infamous streak.

The collection of American players that followed constructed the rest of it.

But for the first time in a while, the state of the drought has a tinge of optimism attached to it.

“I can tell you from just having a tennis academy the last 12 years or so at Randall’s Island that that would be huge for just the energy and the interest level for the sport if one of those guys won,” ESPN analyst John McEnroe told The Post.

There, at the very least, will be a chance of that. Fritz secured his spot in the semifinals with his four-set win against Alexander Zverev on Tuesday.

Then, later in the night, Tiafoe did the same.

So that led to Blackman receiving plenty of messages in the 24 hours that followed — filled with plenty of United States flag emojis — as the USTA’s reality entering the final weekend, something that hasn’t been the case since Roddick cracked the ultimate Wimbledon match in 2009, settled into place.

When Tiafoe and Fritz meet, McEnroe predicted that Tiafoe will have the edge with covering the court.

He’s the better athlete, McEnroe said, and for Tiafoe — ranked No. 20, with Fritz entering as the No. 12 seed — to have a chance to gain an advantage, he’ll need to get Fritz moving around the court.

Fritz, though, is “one of the best ball-strikers I’ve seen in the last 10 years,” McEnroe said, and can control the match with groundstrokes.

Their emergence on this stage has captured the USTA development program paying off, Blackman said.

But it’s not necessarily a relief.

This is Blackman knowing Tiafoe since he was 6 years old and watching him develop.

This is David Nainkin calling Jay Berger to vouch for a young Fritz getting a spot in a Boca Raton clay tournament when spots opened up.

This is Fritz, more recently, maximizing his potential with improved returns and movement and volleys.

This is Tiafoe channeling everything that worked in his run to the Cincinnati Open final last month across his opening five matches in Flushing Meadows.

“The process and the numbers dictate that this is an outcome that we should expect,” Blackman said. “If it came out of nowhere, then it would be like, ‘Oh great. You’d rather be lucky than good.’ ”

In the hours before McEnroe called Jannik Sinner’s quarterfinal match against Daniil Medvedev for ESPN, in perhaps a fitting image given what’s at stake for American tennis the next four days, Roddick walked by.

McEnroe shook hands with the former American tennis star.

They both know what will be at stake Friday.

They both know what’ll be at stake two days later, on the final day of the tournament.

Even if Tiafoe or Fritz leaves Queens on top, it’ll take time for the anomaly to become a new normal, especially with players such as Sinner and Carlos Alcaraz and plenty of others emerging in the sport.

“I don’t think just if one win, all of a sudden everything’s going to change,” McEnroe said, before referencing Roger Federer, Novak Djokovic and Rafael Nadal. “We happened to go through a period the last 15 years, even maybe 20, where the three greatest tennis players that ever lived played at the same time.”

One win, though, would be a start.

The drought would end.

And the wait, finally, would be over.