The Liberty said it would take a 40-minute effort to get past the Mystics.

Turns out it took 45.

In a back-and-forth game of runs, the Liberty made the last one count in a 90-85 overtime victory.

The chippy, frenetic first-round Game 2 win clinched the series 2-0 before a sellout crowd of 9,256 at Barclays Center.

It marked the Liberty’s first playoff series win since 2017, setting up a semifinal date against Wednesday’s Connecticut-Minnesota winner.

But with this star-studded roster and the WNBA’s second-best record, their sights are set far higher.

“It means a lot,” said Breanna Stewart, who had a team-high 27 points and nine rebounds. “I don’t know if we’re thinking about getting this team back to somewhere the Liberty hasn’t been in years, but the first time that we’ve been able to do it as a group.

“There was highs and lows, and we knew it was going to be emotional. Obviously, D.C. is gonna go out swinging and go out with whatever they have, and we stayed the course. No matter what, we didn’t stop playing until the final buzzer, and we came away with the win.”

Stewart bounced back from a subpar 3 of 16 opener with a stellar night.

And with Natasha Cloud harassing Liberty guards Sabrina Ionescu and Courtney Vandersloot, coach Sandy Brondello ran the offense through Stewart late, and the forward — who sat for just 56 seconds all night — had a game-high six points in a 14-9 OT.

“The big moment — and I kicked myself at the end not doing it — but it’s just making Stewie the point guard,” Brondello said. “That was a big moment, because the guards, they just made Slootie and Sabrina, it was a tough night. So we put the ball back in Stewie’s hands, and we’re making better decisions. And that offensive rebound by JJ.”

That would be center Jonquel Jones — also a former MVP — who grabbed Ionescu’s missed free throw with 12.6 seconds left, drew a foul and hit both foul shots to force overtime. Jones added 19 points, 14 rebounds and three blocks to dominate the paint, and held Elena Delle Donne to 11 points on 4 of 16 shooting.

The Liberty had 52 points in the paint — and on a night they hit just 4 of 23 from deep, they needed every one of them.

Washington tried to come out physical, but in the end it was the Liberty that got the two biggest rebounds of the series.

Trailing 76-74 with 27.3 seconds left in regulation on free throws by Cloud (career-high 33 points), Stewart rebounded her own miss.

That set up free throws with 12.6 seconds left by Ionescu, who clanked the first then purposely missed the second to set up Jones’ heroics.

When Cloud threw away a pass for Brittney Sykes, it essentially sent the game into overtime.

“I knew we were going into overtime. And that was really it,” Stewart said. “JJ and I gave each other the look: We wanted to cross and cause some confusion. Sab missed the free throw, and she got it. There was a foul called and I knew she was going to knock down two. And that was really it. We knew that, ‘OK, we didn’t get it done in 40 minutes, but in five minutes, we’ll get it done.’ ”

Cloud’s 3 put Washington up 79-78 in overtime, but the Liberty scored the next six — Betnijah Laney hit a fade, then Vandersloot added a layup and a jumper with 2:29 to play.

Ariel Atkins’ miss from 3 with 8.8 seconds left essentially sealed it, and Stewart’s free throws four seconds later on the other end made it official.

“I definitely think just from the beginning of our whole journey, I felt like this team was special and was going to be able to do a lot of special things,” Jones said.

“I’m a very like step-by-step type of person. I’m just taking one game at a time, one series at a time obviously, and prepare for our next matchup. But I understand that if we played the way we’re supposed to play and do what we’re supposed to do that chances of us being successful are very high.”