BOSTON — A fired-up Kyrie Irving gave the fans the finger and gave the Nets a fantastic outing. But Jayson Tatum gave the Celtics the win. 

The Nets wasted a brilliant performance from Irving and watched Tatum steal Game 1 of this first-round series that felt more like Game 7 of a conference final. 

Tatum’s cutting layup — with Irving defending him — just barely beat the final buzzer and beat Brooklyn 115-114 before a sellout crowd of 19,156 that shook and rocked TD Garden. 

“We’ve definitely got to look at ourselves in the mirror and see where we can control the little things, the details that help us be able to close out the game,” Irving said. “We had the game in control and then a final-second shot beats us and now we’re sitting here asking questions about hostility and s–t like that.” 

Irving had sent back the crowd’s hostility in the form of 39 points, including 18 in the fourth quarter. And after he’d flipped the fans off multiple times, Brooklyn flipped a 15-point third-quarter hole into a three-point lead — one the Nets couldn’t hold — with 45.9 seconds left. 

Irving — who gave the fans the finger after a 3-pointer with 2:18 left in the third — hit another 3 in the final minute to make it 114-111. But Jaylen Brown’s layup cut the lead to one, and Kevin Durant (23 points, but 9 of 24 shooting and six turnovers) missed a potential dagger 3. 

Tatum (31 points) did the rest. Ime Udoka opted against a timeout, and when the Nets blitzed Marcus Smart at the 3-point line, he found Tatum for the winner. 

“I saw two guys flying, so I took a pump fake — actually I was about to throw it to Al [Horford] off the dribble — and I saw JT cut at the last minute,” said Smart. “He made a great play to get the ball off the glass and finish it before the game was over.” 

The play sent the crowd into a frenzy and had the building shaking — the same crowd that Irving had seemingly fed off, sending their bile back as buckets. He also offered a double-finger salute behind his head, and cursed out a fan who’d cursed at him walking into the halftime locker room. 

“It’s the same energy they had for me, and I’m going to have the same energy for them,” Irving said. “And it’s not every fan; I don’t want to attack every Boston fan. But when people start yelling p–sy, and b–ch and f–k you and all this other stuff, there’s only but so much you can take as a competitor. And we’re the ones expected to be docile and be humble and take a humble approach? Nah, f–k that. 

“It’s the playoffs. This is what it is. I know what to expect in here, and it’s the same energy I’m giving back to them. … All’s fair in competition. So if somebody’s going to call me out by my name, I’m going to look them straight in the eye and see if they’re really about it. Most of the time they’re not.” 

The Nets may have lost on the last play, but in many ways the entire 48 minutes showed the flaws they’ve had, their season-long Achilles’ heels not so much exposed but confirmed. 

Battered on the boards. Pummeled in the paint. And beaten when it counted. 

“I feel like we turned it [over]. I had the most turnovers, six of them. That helps them get out on the break. I think the difference probably was, s–t, offensive rebounding. They beat us by one point. It’s just a tough loss, man,” Durant said, shaking his head. “It’s hard to just pinpoint one thing.” 

The Nets got outrebounded 43-29, and outscored 56-32 in the paint despite the absence of Boston center Robert Williams III. 

Bruce Brown had proclaimed the Nets would benefit from having to face Horford and Daniel Theis instead of the injured Williams. Jeered throughout, Brown finished with five points, five fouls and a game-worst minus-13. 

Horford had 20 points, 15 rebounds and a plus-8. 

With the game tied at 63-all early in the third, the Nets surrendered nine straight points. Brooklyn trailed by 15 later in the quarter and had to rally behind Irving, who fed off the venom of the crowd. 

“Embrace it,” said Irving. “Embrace it. It’s the dark side. Embrace it.”