Aaron Nesmith took a transition pass from Tyrese Haliburton and dunked it over Al Horford. The former Celtic galloped around the court and chest-pumped Myles Turner as the rest of his Pacers teammates flooded the court to celebrate an imminent victory.

The Celtics proved they cared about the NBA’s inaugural In-Season Tournament with how they handled their final group stage game last week. They showed that mentality during stretches of Monday’s first ever quarterfinal game of the tournament. But ultimately, the moment meant more for the upstart Pacers.

In a playoff-like atmosphere, the C’s couldn’t get the job done on Monday night in Indiana. Jayson Tatum scored 32 points and Jaylen Brown added 30, but their worst habits haunted them again. They laid another dud in the third quarter. Too many turnovers piled up. And then the Pacers – led by rising superstar Tyrese Haliburton – raised their game to another level in the second half, as they handed the Celtics a 122-112 loss and sent them home from the In-Season Tournament.

The Celtics will not be moving on to Las Vegas for the semifinals. Instead, they’ll return home for a regular season game on Friday night against the loser of Tuesday’s Bucks-Knicks tournament quarterfinal.

Haliburton recorded his first career triple-double – 26 points, 13 assists and 10 rebounds – and brought his best in the second half. He brought them back with a huge third quarter. And then – in a game that featured 16 lead changes and incredible shot-making on both sides – he made the difference late.

Haliburton broke a tied game with a four-point play with 1:33 to go that sent the Pacers on a winning 9-0 run. He hit Buddy Hield for a 3-pointer, then his feed to Nesmith with 44 seconds left sealed it.

Haliburton scored 19 of his 26 points in the second half, which included four 3-pointers. It contributed to 74 second-half points for the Pacers, the most the Celtics have given up in a half this season.

“Just tough shot-making,” Celtics coach Joe Mazzulla told reporters in Indiana. “Shots that he usually makes but he’s a great player.”

The Celtics had a strong start. They held the Pacers – who entered the night with the best offensive in the NBA with a league-leading 128.8 points per game – to just 48 points in the first half. They held a seven-point lead at halftime despite shooting poorly from deep because of their defense.

But then a troubling theme continued – the Celtics lost their way in the third quarter.

Haliburton took over, scoring or assisting on Indiana’s first 20 points of the second half before Bruce Brown’s three-point play tied the game at 71 midway through the third. The Pacers dominated the period and led by 11 late in the third after a 10-0 run that included a pair of treys from Bennedict Mathurin and Hield.

The Pacers outscored the Celtics 37-23 in another disastrous third period for Boston. It all started on offense for the Celtics, who committed six of their 18 turnovers that led to easy points for the Pacers.

“If you turn the ball over, your defense isn’t going to be as good,” Mazzulla told reporters. “So I thought our turnovers led to their transition and I thought we lost our angles as far as our individual defense and gave them some driving lanes and opened up the ability for them to get to the paint. Those two things kind of got us in the third quarter.”

The Celtics did recover from their horrendous third quarter. Sam Hauser kept the C’s afloat with another strong night, drilling one of his five 3-pointers early in the fourth. They went on a 7-0 run midway through the final period, capped by Brown’s second-chance 3-pointer. The C’s dominated the glass with a 56-41 rebounding edge, which included 13 offensive rebounds that led to 20 second-chance points, which kept them alive on a night they shot 29.3 percent from deep.

The Celtics and Pacers traded haymakers down the stretch, and the C’s took a brief 99-98 lead with 4:19 to go. But the Pacers were tenacious and relentless. Haliburton responded with a 3-pointer. He found Nesmith for a go-ahead layup with 2:21 left. After Tatum replied with a jumper, Haliburton found himself in a switch with Brown defending him. As the shot clock winded down, he pulled up and buried an off-balanced triple as Brown sent him to the floor.

It was Haliburton’s big moment. The Pacers’ big moment.

The Celtics finished with 18 turnovers to the Pacers’ six. The Pacers went 19-for-40 from 3-point range, including several big ones late. That helped pave the difference as Indiana moved on to Las Vegas.

“It’s hard to withstand 18 turnovers to six turnovers. So, when you turn the ball over, you just let them get out in transition and I just thought we lost our offensive purpose, didn’t take care of the ball,” Mazzulla said. “When you do that against that team, they make you pay.”