As the men’s semifinals were staged at a rainy U.S. Open on Friday, the American males long had been washed from the scene.

In a disappointing Open performance, none of the U.S. men got to the tournament’s second week and out of the third round. It’s the worst Open showing since 2014.

Though the American men’s tennis drought continued, Martin Blackman, the USTA’s director of player development, assessed the fortnight as a mixed bag.

Blackman sees an upward “trajectory’’ with more grassroots steps being taken to ensure a healthier men’s future picture.

Taylor Fritz (out in the first round), Reilly Opelka (second round) and Frances Tiafoe (second round) entered the Open with career-high rankings. Fritz and Tiafoe are 21 and Opelka turned 22 last week.

“Our goal every year is to do as well as we can at all the Grand Slams and the Open is special because it’s our home Slam,’’ Blackman told The Post. “It wasn’t what we were shooting for. The goal is the second week of Slams and ultimately winning.

“But taking a step back and looking at the trajectory of the young guys we’ve been hopeful about, they all are on a great trajectory,’’ Blackman added. “In the big picture, I’m very optimistic. Three of those guys are in the Top 50.”

The USTA is making money on the Open hand over fist with attendance setting records, partly because of the popularity of the Big 3 (Roger Federer, Rafael Nadal, Novak Djokovic) and the success of the American women led by Serena Williams.

But that won’t last forever, with Williams soon turning 38 and the Big 3 with lots of mileage into their 30’s. Andy Roddick, the last American man to win a Grand Slam, in 2003, made the point that once the Big 3 is history, the pressure on the Americans to produce at Grand Slams will be enormous.

Blackman won’t place a prediction on when he thinks someone from the USTA men’s stable will compete for a major title. He added Jared Donaldson, 22, and Tommy Paul, 22, who lost in the qualifier, as part of the prospect pool as both were hampered by significant injuries in 2019. Paul won the junior French Open four years ago.

“It’s tough to make predictions,’’ Blackman said. “The questions we ask ourselves: Are the players getting better? Are we doing everything we can to help make them better? Does the system work? Are we maximizing and optimizing opportunities for our best juniors. If you’re doing that with a sense of urgency, results will follow.”

The long-running question is why the U.S. women have been so far ahead of the men for so long. Four American women — not including Japanese-American Naomi Osaka — played into the second week. It was only two years ago the Open staged an all-American women’s semifinal.

“It’s not tough to explain,’’ Blackman said. “It’s a challenge we’re constantly trying to solve.’’

Blackman’s favorite response is “the Serena-Venus effect.’’ Young women have outsized role models to follow.

Digging deeper, American girls — and their parents — have more motivation by the more vast amount of financial gain in playing tennis than boys have.

In college tennis, Division I women’s programs dish out eight full scholarships while men’s programs hand out 4 ½, according to Blackman.

“We have to get better male athletes into the game and keep them in the game,’’ Blackman said.

Patrick McEnroe, Blackman’s predecessor, says he can write a tome on disparity between American women and men. Beyond college, young girls have no other lucrative pro opportunities outside tennis. The WNBA, women’s soccer and lacrosse offer meager financial windfall.

“We get the pick of the litter for athletes among young girls,’’ McEnroe said. “We’re not getting those type of athletes for boys.’’

McEnroe noted Coco Gauff’s father and mother played college basketball and track, respectively.

“If Coco was a boy, what do you think would have happened?,’’ McEnroe said.

The implicit answer is basketball, baseball or football would have been the chosen path. In trying to solve the riddle, Blackman hopes the USTA’s relatively new lavish tennis training headquarters in Orlando, Fla. — stuffed with clay and hardcourts — and the new “Net Generation” initiative will kick-start participation.

“Net Generation”, in which Venus Williams and the Bryan brothers Bob and Mike are a spokespeople, boils down to using smaller courts, smaller racquets and softer balls for kids under 10.

“That’s getting kids into the game the right way, fun way with social-skill development,’’ Blackman said.

Better coaching is also a priority. On Jan. 1, coaching requirements will increase for education hours needed certification.

These appear like small steps, but the USTA hopes one day they will erase the misery of American men’s tennis.