In the Liberty’s biggest game of the season, they turned in their worst performance of the year.

Shockingly poor offense and an even worse mindset led to a 78-63 upset loss to the Connecticut Sun in Game 1 of the WNBA semifinals before a sellout crowd of 9,442 at Barclays Center on Sunday.

It was an effort that had the Liberty scratching their heads — and with their backs already against the proverbial wall.

“I wish I knew [why]. We talked about coming out, we came out really slow in the first quarter and then the third quarter. And it’s been happening the last few games,” coach Sandy Brondello said. “It’s mindset, and then frustration sets in, we don’t execute. We know we’re better than that. Now our back is against the wall.”

Against a Sun team they swept four games from in the regular season, they’re in a 1-0 hole going into Tuesday’s Game 2 of this best-of-five series.

The Liberty are in that deficit thanks to a season-low scoring output.

“They were being aggressive, blowing things up for us. But I don’t think we handled it well,” Breanna Stewart said. “We didn’t stay poised or composed. We had the ability to get to the second and third side, and we just didn’t do that.”

Stewart had a team-high 19 points, Jonquel Jones added 14 and 11 rebounds, while Sabrina Ionescu scored 12.

But no other Liberty player cracked double-figures, they were outrebounded, 35-30, despite a glaring size advantage and were stagnant offensively.

“Yeah, we played into their hands. Obviously, they’re a really good defensive team, but we just played right into what they want us to do. Their pressure affected us; we couldn’t really get into a flow,” Courtney Vandersloot said. “I looked up at some point, I’m like, ‘Really?’ We only scored 60 points on our home floor. It’s just not us. But credit to them. They came out and made it uncomfortable for us.”

The Liberty didn’t handle that discomfort.

DeWanna Bonner (20 points, seven rebounds, three blocks) and Rebecca Allen (18 points, seven rebounds, two steals, game-high +23) led a Sun defense that disrupted the Liberty.

“There was one possession three times in a row we didn’t come up with a board. That’s effort, that’s pursuit, that’s taking pride in doing the one-percenters well,” Brondello said. “It was just our energy. We didn’t have the right mindset. So that’s on me. I’m going to make sure I get them ready to play at a high level.”

The Liberty led by three at the break, but allowed nine unanswered points coming out of the locker room.

After leaving Natisha Hiedeman open on a pair of 3-pointers, the Sun run reached 16-2 on Tiffany Hayes’ layup.

The Liberty shot 1-for-5 with three turnovers, got blocked twice and even missed a technical free throw to start the second half.

“We talked about how important the third quarter was going to be, and we just allowed them to get easy buckets,” Jones said. “It has to be a better mindset, better locked-in than what was going on.

“We have to have a more toughness on getting through screens, being able to help our guards when they do get hit. Our defensive coverages just weren’t where they needed to be, whether that was the person on the ball, person off the ball, weak side, all of those things. Ultimately, it kind of drained us a little bit.”

After the Liberty clawed within 53-48, Allen — whom they’d dealt in the offseason trade for Jones — hit a 3-pointer with 3:59 left in the third.

Bonner beat the third-quarter buzzer, then scored the first five points of the fourth to make it 70-55.

“We have to find bodies. We can’t just turn our heads and expect to get the rebound … those are the one-percent things we really need to get cleaned up,” Jones said. “[We need] more energy, more effort. Mindset-wise, it wasn’t there and coming into the next game, we can’t allow that to happen, won’t allow that to happen.”