BOSTON -- At halftime of Game 1 of the Eastern Conference finals, things looked pretty darned good for the Boston Celtics. They'd controlled the paint, taken care of the ball and made shots, and as a result they had a 66-57 lead over the Miami Heat 24 minutes into this best-of-seven affair.

Then the third quarter started. And, yet again, the Dr. Jekyll version of Boston disappeared and the Mr. Hyde version took over.

By the time those 12 minutes had expired, the game had been turned on its head. The Heat had outscored the Celtics 46-25, turning a nine-point deficit into a 12-point lead, and Miami was on its way to a stunning 123-116 win to steal back home-court advantage in this series and leave the second-seeded Celtics reeling from how quickly things got away from them.

"It's a choice. It's a decision," Jaylen Brown said when asked what happened in the second half. "Just come out and play with a different mentality. We came out too cool. It was just almost like we were just playing a regular-season game. It's the Eastern Conference finals. Like, come on.

"We've got to play with more intensity than we did today. We've just got to be better, including me."

If there was something that could have shifted from the first half to the second, it did. Boston had 15 assists and five turnovers in the first half; it had seven assists and 10 turnovers in the second. On the other side, Miami took much better care of the ball, going from nine assists and 11 turnovers in the first half to 11 assists and four turnovers in the second.

The Celtics dominated points in the paint in the first half, outscoring the Heat 40-16. In the second half, Miami edged Boston 24-22. There was a similar change in second-chance points, for which Boston had an 11-2 edge in the first half compared with a 10-7 advantage for Miami in the second.

Sprinkle in the Heat shooting 16-for-31 from 3-point range on the night and Boston found itself, for the second straight series, stumbling out of TD Garden trailing 1-0 to an underdog.

"The only thing we need to adjust to is picking up our physicality and playing some damn defense," Marcus Smart said. "That's the only thing they switched. They didn't change anything from the first half that they weren't doing, they just upped their physicality and that's it.

"There's nothing tactical, X's and O's, it's just come out and guard your yard. They scored 46 in that third, and they got going, and they made us pay, and they led into the fourth quarter."

And while Boston's defense cost the team dearly in the third quarter, it was the offense that fell apart in the fourth. Celtics coach Joe Mazzulla tends to make observations and adjustments with an offensive mindset -- unlike predecessor Ime Udoka, who would typically lean into more physical and defense-oriented lineups. Mazzulla typically goes the other way: smaller, quicker lineups with more spacing and more shooting.

Going to reserve guard Payton Pritchard in both halves Wednesday night over Grant Williams, who was a key part of last year's playoff rotation but has disappeared this year, was a prime example. It also shows up whenever Mazzulla explains what is giving Boston issues in a game, his answers typically centering on problems on offense.

After Boston's strong ball movement in the first half disappeared down the stretch, with the team failing to make a field goal over more than four minutes late in the fourth quarter, Smart -- who had 10 assists in the first half and just one in the second -- said Boston got "antsy," trying too hard to make plays individually rather than collectively.

No one exemplified that better than Jayson Tatum, who had 30 points but didn't take a shot in the fourth. He made three of his four turnovers inside the final three minutes, including on back-to-back possessions with under two minutes to go.

"I turned the ball over, threw it to Jimmy," Tatum said, referring to his turnover out of a Mazzulla timeout with just under three minutes remaining. "That was on me. Doing a shot fake, I just got sped up a little bit. Just got to slow down a little bit in those moments."

Mazzulla chose not to call any timeouts in that disastrous third quarter, reviving a season-long storyline around his penchant for letting his players sort things out themselves.

"I called two in the first quarter," Mazzulla said when asked about his lack of timeouts in the third.

When asked again, he said, "Don't call two in the first quarter. Save it for the third-quarter run."

Smart defended Mazzulla, saying it's not on the coach to bail the players out.

"Joe is real big on a lot of times not bailing us out on stuff when we're playing like s---," Smart said. "So, we've got to look ourselves in the mirror. Joe can call a timeout, and then what? We come out and do the same thing? It's on us.

"Joe and his coaching staff, they put in a lot of work to come up with a game plan and put us in the right spots to succeed, but they're not out there playing. We've got to come together and we've got to start helping each other out on both ends."