After taking one of the most humiliating beatings in team history, Jordi Fernández challenged his Nets to show more fight.

Fernández saw plenty of fight Friday. And then saw it undone by a couple of endgame gaffes that cost Brooklyn a 130-126 double overtime loss to the Celtics in front of a sellout crowd of 17,727 at Barclays Center.

The Nets earned a five-point lead with just five seconds left in overtime. That’s when their own mental mistakes did them in.

“We’re up five, they throw a cross-court pass and you have a chance to foul…and then probably make one (and) it’s over. We didn’t do it and they made the 3. Then the other one is we’re around the 3-point line, there’s a miscommunication and they get a wide-open shot,” Fernández said. “We made a couple mistakes, and those mistakes in this league against a very good team makes you pay.”

Boston did just that.

Up 117-112, they didn’t intentionally foul, and let Payton Pritchard (game-high 32 points) drill a 3-pointer with 4.5 seconds left.

Then, after rookie Nolan Traore (21 points) missed one of two at the charity stripe to give the Celtics one last gasp, a defensive miscommunication left Hugo Gonzalez too open for a game-tying corner 3-pointer with :00.4 left.

That forced a second overtime, where the Nets ran out of gas.

“He was just wide open. He was wide open. He hit the shot,” said a dumbfounded Nic Claxton, who had 18 points and nine rebounds. “I’m confused. I got to watch it. I’m confused. I don’t know how it got so open. That’s in the corner. I don’t know. I don’t know. I can’t give you an answer for thatr.”

Michael Porter Jr. — who had a team-high 30 points and eight boards — ended up miscommunicating with Noah Clowney. He passed off Gonzalez to the young forward thinking Clowney was going to cover him.

But with one player thinking man and the other thinking zone, the Nets were in abysmal disarray and got punished.

“Obviously, it was poorly executed. I tried to pass the guy through to Noah on that side, and we just weren’t all on the same page,” Porter said. “Coach Jordi was trying to communicate on the sideline, it was just such crunch time, we didn’t know we were still zoning up, or if we were in man…We were just discombobulated. We got to learn from it, hopefully do better in the next situation.

“We just got discombobulated. But yeah, it’s on us, the players, to communicate, and obviously the No. 1 thing is don’t give up a wide open corner uncontested 3. So somebody’s got to get him, whether that’s me following him through to the corner or the guy on the inbound kind of taking that zone of the area away. We’ve just got to make it happen.”

The Nets (12-31) have dropped 12 of their last 14, the worst a 120-66 loss Wednesday to the Knicks. It was the second-largest margin of defeat in franchise history, and their worst output since a 90-65 loss to Miami on March 12, 2005. To a man, they’d acknowledged the need to redeem themselves.

Brooklyn came close, leading by as many as 11 points, but it couldn’t hold it.

Traore had 21 points, playing 36:51 and getting key minutes instead of Egor Dëmin down the stretch because of his ability to touch the paint.

And Ziaire Williams (14 points, eight rebounds) played feisty defense.

Pritchard (32 points) and Jaylen Brown (27 points, 12 assists, nine rebounds) led Boston.

But in the second overtime, Brooklyn finally ran out of gas.

The Nets (12-31) stayed fifth in the lottery race, 1 ½ games behind fourth-place Sacramento and moved two ahead of sixth-place Utah.

But they’re now only 2 ½ behind Indiana for a spot in the top 3.