It’s been over two decades since an American man won a Slam. But as Frances Tiafoe and Ben Shelton prepare for Friday’s all-American showdown at the U.S. Open, they say there’s an opening to finally break through.

“There’s a window; there’s an opening in the game,” Tiafoe said. “[We had] a lot of familiar faces winning the event. We had three greats playing at the same time and winning events. … I think this is definitely the one that we can win. We just haven’t done it yet. We have five guys in the top 20. All guys are dangerous. All guys are capable. It’s kind of a why-not thing.”

Since Andy Roddick’s 2003 U.S. Open crown — the last Slam by an American man — that Big 3 of Novak Djokovic, Rafael Nadal and Roger Federer monopolized the game.

From 2005 to ’14, a stunning 38 straight Slam finals featured one of them, and only eight finals since Roddick’s Flushing win did not include one of the trio.

But Tiafoe and Shelton — whose third-round tilt is a rematch of last year’s quarterfinal — are two of five U.S. men ranked in the top 20.

That’s a clear uptick among American men, one they’re convinced will continue.

“The quality of U.S. men’s tennis is 100 percent on the rise,” Shelton said. “This is the most guys we’ve had in this competitive position in a while. … Whenever somebody has that big push, it puts the other guys on alert and makes them play even better. When I’d become the No. 1 American for 4 ½ minutes, Taylor Fritz got hot.

“Everybody wants that big next Grand Slam, next big moment. But the gradual improvement and progression is what’s really going to put Americans more towards the top of the game.”

That progression will only be helped by moments like Friday.

Shelton earned a 6-2, 3-6, 7-6 (7), 6-2 win in last year’s quarterfinal before falling to Djokovic.

But two exciting young players meeting on a big stage stateside is what American tennis needs to reach fans of the NBA, NFL, etc.

“Frances is the best in our sport in recent years on the American side of reaching that casual sports fan,” Shelton said. “He’s got all the hooper friends who come to his matches, travel to see him play, the music industry. He’s done a great job putting American tennis on the map in different sports and making it cool again.

“It’s really important that our sport continues to grow, and that’s reaching young people and fans who’re not necessarily tennis fans but just casual sports.”

Both have outgoing personalities that help in that: Shelton with his showmanship and Tiafoe with a megawatt smile and pop culture crossover appeal.

“We’re both kind of goofy,” Shelton said. “He definitely knows how to bring out the clown side of me. I do the same with him.”

But both are dialed in at the Open, and each has grown of late.

Tiafoe has benefitted from hiring David Witt, who coached Venus Williams and Jessica Pegula.

He went from 15-15 this year before hiring Witt to 11-4 since.

And Shelton, blessed with the game’s biggest serve, has become a better mover, returner and thinker.

In the past, his arm has been sore by the second week of Slams after repeatedly reaching back for 150 mph. Now, he has more left in the tank.

“I’m getting through my service games this year without having to light up the radar gun to do it,” Shelton said. “Everyone in the crowd is screaming 150, 145, 150, but I found a really good groove maxing out at 134, 135 mph.

“If I can stay a little bit lower, I don’t have the shoulder soreness or pain I had last year. It’s really important because it was something that was coming up for me when I got to the second week of Slams. … But I’ve gotten better at hitting spots. If I hit a spot at 135 or I hit it anywhere in the box at 147, it’s often the same result.”

Both will put their growth on display Friday in a match that can only help American tennis.