Not one but two defensive collapses Sunday left the Nets beaten and coach Jordi Fernandez steamed.

They took their feet off the gas at the end of the third and again at the end of the game, and they ended up crashing to a 118-113 loss to Giannis Antetokounmpo and the Bucks before a sellout crowd of 17,926 at Barclays Center.

A 12-point cushion with three minutes left in the third, a five-point edge with three to play, and a monster night from Dennis Schroder all were wasted.

“No. 1 thing: My mistake of keep doubling Giannis,” Fernandez said. “Towards the end when we had a two-possession game, probably we had to stay home and try to guard one-on-one. I don’t know if we could, but at that point probably that would’ve been the best thing to do. So I take responsibility for that; [it] opened up the 3-point line, and they made them.

“But the reality is, in the last three minutes of the third and the last six minutes of the game, it was a minus-19 for us. Our defense was atrocious. And if our guys who wear a Brooklyn Nets jersey don’t play harder defense, they don’t have a place in our roster. And they know it. They take pride on defense. … So that’s how you lose games in the NBA. We didn’t guard anybody in the last three minutes, last six minutes; last three of the third, last six in the fourth.”

Dennis Schroder had 34 points, 11 assists and no turnovers, and he held Milwaukee All-Star Damian Lillard to a quiet 15. But the Bucks’ other star, who has long been a target of the Nets, dominated.

Antetokounmpo had 34 points, 11 rebounds and huge plays down the stretch, either scoring or drawing doubles to create shots for teammates. And they drilled them, shooting .574 from the field and .484 from deep.

The Nets led 83-71 after a Day’Ron Sharpe free throw, but Antetokounmpo had seven points in a 15-3 run over the last 3:03 of the third to tie it entering the fourth.

Nic Claxton (21 points, 10 boards, three steals, two blocks) helped the Nets rebuild that cushion. He blocked Khris Middleton and Antetokounmpo at the rim and ignited the crowd. And Cam Johnson (26 points, 4-for-8 from deep) hit a step-back 3 to give the Nets a 106-101 lead with 3:47 to play.

But they surrendered a 17-7 run to close the game.

“Just letting them get looks that they wanted, and let them just be too comfortable,” Johnson said. “The rest of the game we did a really good job. But in those two parts of the game, we could’ve been a lot better.”

The Nets tried to trap Antetokounmpo and paid for it. The nets saw the Bucks hit four of their last five shots, and 2 of 3 from deep.

Middleton — in only his second game of the year — hit a 3-pointer to spark the run, and Antetokounmpo beat a trap to find an open Bobby Portis for a go-ahead 3 with 2:08 left. The Nets trailed 109-108 and never led again.

“I mean, [Fernandez], he took accountability, and that’s good. But at the end of the day, it’s on the players. It’s on us out there to play with as much energy as possible,” Claxton said. “And there definitely was a dip at the end of the third and pretty much for most of the fourth quarter. So, we have to be better.”

The Bucks (12-11) struggled out of the gate this season, prompting questions of whether Antetokounmpo would ask out. But after a 2-8 start, they’ve won 10 of 13, including snatching victory away from the Nets on Sunday.

“Coach, he tries to take accountability, but as a team, we also made some bad decisions late in that game,” Dorian Finney-Smith said. “Some of them traps, we had them stopped; there was no reason to even trap. That gave them wide-open transition 3s after we scored buckets. You can’t do that when you’re trying to beat a playoff team.”