NEW YORK — The young, upstart Detroit Pistons, who had more than tripled their regular-season win total and were seeking their first playoff win in 17 years, seemingly had little else to prove in Saturday’s first-round opener.

They had shown to be up to the task of defending All-NBA guard Jalen Brunson, who had started an ice-cold 2-for-13 from the field. Detroit looked fully in rhythm on offense and was shooting better than 50% from deep through three quarters to lead by eight heading into the fourth.

But all of it — the defense on Brunson, the Pistons’ offensive rhythm and just about everything else — came crashing down for Detroit at Madison Square Garden in the fourth quarter.

The Knicks, whose pressure defense forced a pair of turnovers to open the final period, turned the momentum quickly and embarked on a 21-0 run over a four-and-a-half-minute span to run away with the contest 123-112 and take a 1-0 lead in the best-of-seven series.

That fateful fourth quarter began on the wrong foot for the Pistons, who got whistled for a five-second violation when trying to inbound the ball on the first play of the period after Knicks guard Cameron Payne unexpectedly applied pressure in the backcourt.

If that wasn’t bad enough, Detroit committed a 24-second shot clock violation on the following possession. And talented second-year wing Ausar Thompson blew a breakaway dunk in embarrassing fashion about a minute after that.

The mistakes opened the door for the Knicks — who to that point had been kept afloat by OG Anunoby’s offense — to begin making up ground.

“Things turned really, really quickly,” Brunson said, acknowledging that the Pistons’ five-second violation felt like the catalyst. “I think we were just connected in the way we were getting stops out there. The way we played defensively was key for us.”

As all of that was happening, the Knicks and their fans were tapping into the momentum they had generated from the Pistons’ turnovers. Karl-Anthony Towns (23 points, 11 rebounds, four steals) scored five quick points immediately after Detroit’s miscues. Payne, who forced the initial violation, was solid in the fourth, scoring 11 of his 14 during the period.

And Brunson, arguably the NBA’s best closer this season with a league-best 52 baskets in the clutch, found a comfort zone after returning from what looked like an ankle tweak to take over in the final period.

Brunson, who recently missed nearly a month of action due to a right ankle injury, left the game and bench area briefly. He appeared to be hobbling after tweaking it during a play late in the third. When he returned, he had switched from the lime green sneakers he had to an off-white pair. Coincidence or not, he shot 5-for-7 for 12 points and had three assists in the fourth.

He downplayed any potential injury and denied that the shoe change was in any way responsible for his late-game turnaround. Coach Tom Thibodeau said Brunson might have had another form of help, though. “I think he grabbed his cape,” Thibodeau said with a smile.

Understandably, the Pistons — who haven’t won a playoff game since 2008 and saw their playoff losing skid extend to 15 games — weren’t smiling in their postgame interviews. Coach JB Bickerstaff also sounded concerned about the status of backup big man and enforcer Isaiah Stewart, who left the game with an apparent knee injury and didn’t return.

But Detroit wasn’t all doom and gloom about the defeat, necessarily.

After a rough start, Pistons All-Star Cade Cunningham settled into something of a rhythm midway through the game to finish with 21 points and 12 assists to go with his six turnovers.

Veteran wing Tobias Harris, who had a 22-point first half Saturday after having gone scoreless in an elimination game against the Knicks last season with the Sixers, stated the obvious following the contest. For as ugly as the meltdown was, Game 1 was ultimately just one game.

“For the majority of it, we did a really good job staying composed out there,” Harris said of the Pistons, who beat the Knicks three times in four outings during the regular season. “It’s just one game of a series. Playoff basketball is about how you respond. [How you respond] is one of the biggest things in this league. I’m excited to see how we respond.”