BOSTON — Jordi Fernández insisted all those early losses didn’t turn the Nets into losers. Then they went out and earned their biggest victory of the season.

Brooklyn ground out an impressive 113-105 NBA Cup win over the Celtics before a sellout crowd of 19,156 at TD Garden, seizing a lead and never giving it up.

Michael Porter Jr. was the Nets’ closer, with a game-high 32 points, including the Nets’ final 14 of the night. Center Nic Claxton had his first-career triple-double (18 points, 12 assists and 11 rebounds), and rookie Egor Dëmin added 12 points, six boards, five assists and the biggest basket of the evening.

After building and then blowing an 18-point cushion — watching it shrivel to just 96-94 — Dëmin’s 3-pointer staunched the bleeding.

Then Porter reeled off Brooklyn’s final 14 points.

“It got dicey, right? Because they were down and they fought back,” Fernández said. “We took all those punches, and then we punched back. It goes from Egor’s big shot and then Mike towards the end going into video-game mode. But the reality is we got enough stops and rebounds to survive their punch.”

Tank-happy fans will call the victory Pyrrhic. Brooklyn (3-12) fell to fifth in the lottery odds, now a half-game behind the fourth-place Kings.

But after avenging Tuesday’s defeat to Boston, the Nets won’t care.

“We lost to them at home. We just wanted to get our licks back,” Claxton said.

The Nets avoided elimination in the NBA Cup and beat a good team for the first time this season.

“There’s a difference between losing and being a loser. Losing is not getting the outcome that you want. Being a loser is when you lose and you don’t care, or when you’re not willing to [do] whatever it takes,” Fernández said. “Our group now has embraced that.”

And Friday that started to pay dividends.

An 11-2 run in the second quarter gave the Nets a lead they never surrendered, even though they had chances to against Boston (8-8).

“The perfect word is growth — as a collective, but individually as well,” Porter said. “We’re a young group, but there’s growth. And if in the fourth quarter when things get tight like that on the road and we can execute and get good shots, we’re going to win a lot more games.”

That won’t please the pro-tank crowd. But the Nets are clearly showing improvement.

With the score knotted at 51-all on Derek White’s 3-pointer with 3:23 left in the first half, Brooklyn closed on an 11-2 run.

The Nets had four baskets in the spurt, with Dëmin scoring two — a turnaround bank shot and driving layup — and assisting the other pair, a Porter 3-pointer and a layup by Noah Clowney (19 points, three blocks) with just 2.8 seconds left.

The latter gave Brooklyn a 62-53 lead at the break, and they never lost it thanks to withering defense in stark contrast to their sievelike tendencies earlier in the season.

Clinging to a 74-70 lead with 5:18 left in the third, the Nets mounted a 16-2 run. They held the Celtics to 1-of-8 shooting with a pair of turnovers, the second coming on Drake Powell stealing a Sam Hauser pass and finding Ziaire Williams for a dunk.

That gave the Nets a 90-72 lead with 1:18 left in the third. They spent the fourth trying to hold on to it.

Jaylen Brown (26 points) led Boston back, and the Nets saw their cushion shaved to just 96-94. But Dëmin showed he wasn’t cowed by the moment, with a right-wing 3 to stem the tide.

And up 99-95, Porter played closer by scoring Brooklyn’s last 14 points.

After going 1-of-6 in the fourth quarter in Orlando — where the Nets blew a 16-point cushion — this time Porter was 7-of-8 in the final period.

“It wasn’t just the Orlando game that I felt like I wasn’t as aggressive as I should’ve been in the fourth. It was the last game against the Celtics too,” said Porter. “So [on Friday], I didn’t want it to be déjà vu. I said I’m gonna go in there and be aggressive and luckily shots fell.”