Ben Stiller’s first instinct was to call the Knicks’ historic NBA Finals comeback on Wednesday night unbelievable.

But the celebrity superfan caught himself. Because he can believe it, as he’s has a front-row seat to an entire Knicks playoff run in which they have proved their resilience and grittiness time and time again.

So while a 29-point comeback and OG Anunoby’s all-timer of a putback with 1.2 seconds left to put the Knicks one win away from ending their 53-year championship drought seems unfathomable to many, it isn’t to the 60-year-old actor.

“What a team, what a team. Come on,” Stiller told CBS New York after the Knicks took a 3-1 series lead over the Spurs. “I can’t believe — I mean, I do believe it because this team…”

Stiller then trailed off, still in euphoria after the largest comeback in NBA Finals history.

“Most insane comeback I’ve ever seen,” concluded Stiller, who is filming an HBO documentary on his phone during the Finals, per Page Six.

Stiller and his wife, Christine Taylor, were among a litany of celebrities at MSG on Wednesday who went from completely deflated at halftime to manic celebration.

There were the usuals like Spike Lee, Fat Joe, Timothee Chalamet, Tracy Morgan and Mariska Hargitay, loyalists like Chris Rock and Adam Sandler, and pop-ins like Taylor Swift and Hailey Bieber.

For some of them, they’ve been waiting for this moment for most, if not all, of their lives. Stiller was among the celebs at MSG for the Knicks’ last Finals appearance in 1999 — also against the Spurs — along with Lee, Rock and Sandler.

Stiller told The Post’s Stefan Bondy on Sunday that he has dealt with “a lot of pain” as a Knicks fan since their last title in 1973.

“My whole life I’ve lived with this idea that, ‘Oh, the Knicks aren’t going to make it, the Knicks have never been good, the Knicks have been a joke for a long time back in the day,’ ” Stiller told The Post when the Knicks led 2-0 in the series. “It almost became something you accept.”

When the Rangers ended their 54-year Stanley Cup drought in 1994, the voice of the team, Sam Rosen, famously said “this one will last a lifetime.”

One more win, and the same will be true for the Knicks.