Jaylen Brown’s improbable shot helps Celtics to a thrilling 133-128 overtime victory, and other observations from Game 1 vs. the Pacers

But after the Pacers threw away an inbounds pass, Brown, who had not made a 3-pointer all night, lined one up from the left corner and drilled it with 5.7 seconds left, helping send the game to overtime, where Boston rode its momentum to a 133-128 win.

Jayson Tatum had a rough fourth quarter and start to overtime before pouring in 8 points in the final 2:27 of the extra session to finish with 36 points and 12 rebounds. Brown had 26 points and 3 of them will be remembered for years to come. Jrue Holiday had his finest game as a Celtic, tallying 28 points, 8 assists, and 7 rebounds.

Tyrese Haliburton had 25 points and 10 assists to lead Indiana, which shot 53.5 percent from the field.

The Celtics coughed up a 12-point first-half lead, stormed back to take a 13-point third-quarter advantage, only to lose that, too. Pascal Siakam hit a mid-range jumper before coming up with a steal and dunk that gave Indiana a 108-104-lead with 4:33 to play in the fourth.

Every time the Celtics hit a potentially momentum-swinging shot over the next few minutes, the Pacers had an answer. Two free throws by Brown pulled the Celtics within 115-114 with 57 seconds left, but Andrew Nembhard answered with a tough 19-footer after Al Horford switched onto him, a trend down the stretch.

After a timeout, Tatum lined up a wide-open 3-pointer but it was off. Haliburton dribbled upcourt and somehow simply coughed up the ball despite no defender even bothering him. But Boston was unable to capitalize. Derrick White drove and missed a layup, and Tatum’s mid-range putback attempt was off.

The game seemed just about over there, but after Haliburton was fouled, the ensuing inbounds pass was kicked out of bounds, giving Boston one more chance with 8.5 seconds to play. Brown caught the ball in the left corner, faked twice, and threw in a dart that tied the score with 5.7 seconds left.

Haliburton struggled to find space for a game-winner and his heave from the right arc at the buzzer never had a chance.

The Celtics led, 121-120, with two minutes left in the extra session when Tatum coughed up a turnover and the Pacers raced the other way. Derrick White lined up a block attempt on Haliburton but fouled him instead, and he hit all three free throws to put the Pacers back in front, 123-121.

The Pacers went small after Aaron Nesmith fouled out, and Tatum scored inside as he was fouled by T.J. McConnell. He made the free throw to give Boston a 1-point lead, and after another Haliburton turnover, Tatum calmly stepped into a 3-pointer and finally connected, making it 127-123 with 42.8 seconds to play.

Siakam then committed the Pacers’ 22nd turnover of the night, and Indiana faded away.

Observations from the game:

▪ The first two rounds of the playoffs at TD Garden just didn’t feel like the playoffs. It was partly because of the undermanned opponents, partly because the Celtics lost Game 2 both times, and partly because there were few tense moments. But the start on Tuesday felt more like vintage TD Garden. Fans were standing at their seats long before tipoff, and the buzz at the opening tip was palpable. Welcome back, playoffs.

▪ The Celtics, of course, have made conference finals appearances near annual occurrences. This is not new. But the same cannot be said of the Pacers, and during the first four minutes Indiana’s nerves seemed to show.

The Pacers missed their first six shots, committed a pair of early turnovers, and stumbled around on defense as the Celtics surged to the rim with little trouble. Former Celtic Nesmith appeared particularly out of sorts. He coughed up both early turnovers, including Tatum’s steal on the first possession that led to a thunderous Brown dunk, and then missed a relatively easy layup and left a wide-open 3-pointer short.

The Celtics held a 12-0 lead less than three minutes into the game.

▪ Coaches generally condense their rotations when the playoffs arrive, but Indiana’s Rick Carlisle continues to roll with his bench groups. Two minutes into the second quarter he already had used 10 players. But the approach is understandable given how productive his second unit has been.

Three subs came in less than five minutes into the game, with the Pacers still reeling a bit, and helped steady Indiana. Obi Toppin had an especially productive stint, going 5 for 6 from the field.

▪ For most of the playoffs Tatum has played the entire first quarter and started the second before getting a brief break. His sub pattern Tuesday resembled the regular-season approach, with brief breaks in each quarter.

Tatum appeared to be pressing a bit in the first half, determined to put an early stamp on the game. But that led to a few awkward isolation situations after the Celtics had seized control with such a big start. In the second quarter he missed a fadeaway by about a foot, had a 3-pointer blocked by Isaiah Jackson, and committed an offensive foul when he tried to clear space by pushing off. Like most of the Celtics, he did have success getting to the rim within the flow of the offense, although he missed both of his first-half 3-pointers.

The Pacers don’t switch on screens nearly as often as a team such as the Heat did, so hunting mismatches is not as clear.

▪ Tatum did show good awareness on one second-quarter possession. He passed to Horford near the left arc, and Horford quickly started looking for a path to get the ball back to Tatum. But Tatum saw Horford was being guarded by Jalen Smith and motioned to Horford to attack the rim. The Celtics big man wasted no time backing down Smith with a few powerful dribbles before scoring inside as he was fouled.

▪ The Pacers showed in their Game 7 win over the Knicks on Sunday that their offense remains flammable. After their 0-for-6 start, the Pacers closed the half by making 27 of 40 shots (67.5 percent), including 8 of 15 3-pointers.

Myles Turner was dominant. The Pacers big man had his way inside and out, with 18 first-half points on 7-of-10 shooting, along with four assists. The Celtics did not really miss Kristaps Porzingis against the Cavaliers, but this was a situation where his rim protection would have been valuable. The Pacers mostly ignored backup center Luke Kornet at both ends of the floor in the first half.

Indiana never led in the first half but tied the score several times late in the second quarter, including when Haliburton’s 39-footer sent the game to halftime knotted at 64.

▪ The Pacers took their first lead in the opening minute of the third quarter, and there were some nervous groans when a Haliburton 3-pointer gave Indiana a 69-64 lead, its largest. But the Celtics had to feel good about the fact that with a quick pass and cut they could mostly get whatever shot they wanted against this defense.

The Celtics responded with a 23-6 surge that was capped by a Holiday 3-pointer for a 90-77 lead, their largest, with 4:15 left in the third.

This is usually the point that teams crumble against this dominant Celtics squad, but the Pacers were unbothered. They closed the quarter with a 16-4 burst and pulled within 94-93 when Haliburton leaned and banked in a 3-pointer at the buzzer. Tatum was on the bench for most of that surge.

▪ The Pacers’ defense is not great, but it is built to take away 3-point looks. It appeared that the Pacers were focused on forcing Horford to beat them from long range. Through three quarters he had attempted nine 3-pointers, while Tatum and Brown had taken just four combined.

▪ Holiday emerged as Boston’s third scoring threat. He hit four 3-pointers over the first three quarters and did plenty of damage by backing down smaller defenders and scoring inside. The Celtics needed all of it.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him @adamhimmelsbach.