For the 35,000 tennis fans streaming daily into Flushing Meadows, there are several US Open traditions that stand the test of time: swallowing the sticker shock over a $7 bottle of water, cheering for rival Eastern Europeans in a fifth-set showdown — and watching eager teenage ball boys take to the court in earnest.

But on Court 14 during a late-afternoon slugfest on opening day, the ball boy jumping up to catch the cross-court winner wasn’t some 16-year-old string bean, but a tattooed rookie three times his age.

A recently retired undercover narcotics specialist and grandfather of three, 53-year-old Todd Reed should be lazing in the stands, not hustling as a rookie ball boy.

But Reed likes to defy the odds: Not only is he the oldest ball boy at the USTA Billie Jean King National Tennis Center this year, he’s a war veteran, a Green Beret and an amputee who lost part of his right leg after a land mine explosion in Iraq during Desert Storm.

And his patriotic red, white and blue prosthetic is stealing the show.

“I noticed him right away,” says attendee Carol Liguori, 54, of Toms River, NJ, during an afternoon women’s singles match. “Usually you see young boys who are 16 — then I saw his leg. I give him a lot of credit!”

Over at the ball-boy hangout known as “the perch,” Reed’s colleagues are poring over their iPads between matches and dissecting the MTV Video Music Awards.

They’re also embracing the shake-up on court.

“I love that he’s here — I think it’s really cool and inspiring,” says fellow ball boy Corey Stella, a 19-year-old college student. “I already worked a match with him. He can definitely keep up and has no problem fitting in.”

The 275 ball boys employed at the US Open make eight bucks an hour — plus a daily food stipend — and their duties include P. Diddy-esque umbrella-holding for players during breaks, fetching bananas, ice, towels or anything else a player needs, and never, ever keeping the player waiting.

The youngest of the bunch is 14 years old.

Reed, on the other hand, is almost four times that age and sports six tattoos to prove it, including a Special Forces crest on his left forearm and a Wounded Warrior Amputee Softball Team inking on his left leg.

So how does it feel to be this year’s oldest US Open ball boy?

“Ball person,” corrects Reed. (About one-third are women; the oldest is in her late thirties.)

A North Carolina-based sports fanatic who plays outfield on the Wounded Warriors team, Reed landed the new gig from a USTA military initiative. His wife, Renee, convinced him to jump at the chance.

“I told him this is a once-in-a-lifetime opportunity — that he’s probably going to be the oldest ball person in history!” she says.

Close.

Reed is the second-oldest ball boy to work the US Open — bested only by a 65-year-old lawyer, Jerry Loughran, who quit the courts last year.

“A lot of [my current duties] draw on my past experience in the military,” says Reed, clad in head-to-toe Ralph Lauren, an upgrade from drab Army fatigues. The decorated sergeant first class says the similarities include working as a unit — in this case, the five other ball boys on court — to complete a mission.

“You have to be calm under stressful situations, not get flustered,” says Tina Taps, director of ball persons, who hired Reed based on a video submission and phone interview. “It’s a natural fit for someone like Todd.”

Still, it’s a long way from that crystal-clear morning in April of 1991, when Reed, on his second tour, was on a recon mission in Mosul in northern Iraq.

“I took a step, and as I transferred from my right to left foot, [the land mine] exploded. It blew my foot right off then and there,” recalls Reed, who went flying six feet — all while conscious.

He spent six weeks in recovery at Walter Reed Army Medical Center in Washington, DC, followed by six months of rehabilitation.

And yet he’s still amazed at the latest turn his life’s taken.

“Here I am at the US Open in 2014. If someone told me back then that I was gonna be a ball boy, I’d say, ‘Keep dreaming!’ ”

Plus, there’s the possibility of bumping into Roger Federer — or, even better, working his match. It’s Reed’s dream to meet the legend.

“I’d love to be on [the court at Arthur] Ashe [Stadium]. There’s talk it will happen on Labor Day,” he says, adding that he’s not getting his hopes up. After all, it’s mostly seasoned ball kids who get to work the marquee matches, based on experience and skill.

But whether or not he makes it onto the main court, life is good for Reed, who is next off to Japan to play for his softball team.

“I think the good Lord knocked me down a few steps. I was pretty cocky back then,” Reed says of his younger Army days. “I think ‘this’ happening enriched my life in many ways. I’m blessed — everything else is a bonus.”