Reilly Opelka is familiar with the Flushing Meadows grounds, but only when admission is free during the U.S. Open qualifier.

The new American man to watch, the 6-foot-11 Opelka, will make his U.S. Open main-draw debut Monday against No. 11 seed Fabio Fognini when the tournament kicks off about 1 p.m.

Court 17 — the fourth showcourt — should be buzzing. Ranked 42nd, he is the tallest player in tennis history with a 140-mph serve and mobility that the 6-foot-9 John Isner always wished he possessed.

Opelka, who turns 22 Wednesday, has played in three straight Open qualifiers but never got through to the main event. Now his ranking is plenty good enough for the main draw. He can’t help but wonder if the USTA could have given him a wild-card boost in the past.

At 18, Opelka was ranked as high as 110 and never got a wild card here. Without naming names, he felt slighted.

“It’s a nice feeling, especially because I earned my spot,’’ Opelka told The Post on Sunday. “Even though I’m an American, I never took a wild card. Plenty of guys had wild cards here and never earned it. It’s a better feeling know I didn’t get picked. I should have.

“They gave it to guys behind me. It’s all political.”

Desperate for a men’s star, the USTA is banking on Opelka emerging as that guy — perhaps becoming larger than Isner, who never made a Slam final.

Opelka, a Chicago Bears fan from Michigan who moved to Delray Beach, Fla., has a hot streak in the city. He won the New York Open at Nassau Coliseum in February.

However, he has a sizable task in facing the great Italian service returner in Fognini, who upset Rafael Nadal on these grounds in 2015. It stands as the most compelling day-card match.

“I have a tough draw,’’ Opelka said. “It sucks. Everyone’s good here, but it could’ve been better, could’ve been worse. I could have had Novak [Djokovic].’’

Opelka’s parents are tall, so his growth spurt that started at age 13 and ended at 17 was expected. “I grew a ton,’’ Opelka said.

Opelka told The Palm Beach Post this week he grew a quarter-inch each month from the ages 13 to 17. Because of the way he moves, Opelka’s basketball height is only an advantage — from a service standpoint and potential at the net. As Andy Roddick said this week, “You can’t teach 6-11.”

“I do move better than Isner, but he serves better than I do,’’ Opelka said. “He’s got the greatest serve of all time. Isner has had a lot of success. There’s more negativity about him, but being top 20 in the world for seven years, he just played in the wrong era against four of the greatest players of all time.’’

Opelka said fans need to be patient before labeling him a Grand Slam championship contender. The trend is for men’s players to peak closer to the age of 30, he said. Indeed the average age of the top 100 men has risen dramatically.

“I think my best tennis will come closer to 27, 28,’’ Opelka said. “It’s just preparation for that and learning experience. I hope my time comes sooner but I don’t expect it.”

He also doesn’t expect to ever need a wild card from the USTA.

“Hopefully that’s in the past and don’t have to go that route again,’’ Opelka said. “I earned my spot and wasn’t just here because I’m an American with a wild card.’’