The ambient pop of a catcher’s mitt greets you before you see the interior of the Lab. Inside the unmarked door on St. Nicholas Avenue in Harlem, past the canvas tarp blacking out the windows and around the ladder serving as a makeshift camera stand, awaits an 80-foot-long pitching mecca where the outside world quickly disappears.

The hurried passers-by might think this place amiss, but the former shoe store two blocks from the Apollo Theater and beneath a Chuck E. Cheese’s feels just right to the ballplayers who trek uptown to hone their craft.

On a Wednesday in January, that group included Adam Ottavino, the facility’s gatekeeper, host and a major league reliever for the past seven seasons; four minor league pitchers; and one Baruch College catcher. Ottavino’s father, John, an actor based in Brooklyn, is also there.

“These guys are just a bunch of big kids,” said the older Ottavino, gesturing to the men aged 22 to 33. “I come in and clean the place once a week. If they can’t find something, I usually know where it is.”

On this day, that meant finding a misplaced blue lacrosse ball that his son uses for warm-up exercises. On any other Wednesday, Adam Ottavino would be throwing off the portable pitching mound, but on the eve of the day his life would change, there was a bit too much at stake.

“Something good might be happening soon,” a coy Ottavino told the five fellow ballplayers who hope to follow in his footsteps. “I can’t get injured today.”

The next day, Jan. 17, Ottavino, a 33-year-old right-hander from Park Slope, completed a three-year, $27 million contract with the Yankees.