INDIANAPOLIS -- After missing Saturday's win over the Miami Heat with an upper respiratory infection, Indiana Pacers superstar Tyrese Haliburton didn't look like himself in the first half of Monday's in-season tournament quarterfinal against the Boston Celtics.

"The first half, I was dead," Haliburton said later.

So, at halftime, Haliburton -- at the behest of the athletic training staff -- did something he's never done before: He used an inhaler.

When the second half started, he looked like himself again -- and the Pacers looked like a totally different team. Behind Haliburton's first career triple-double -- 26 points, 10 rebounds and 13 assists in 40 minutes -- Indiana emerged with a 122-112 victory over the heavily favored Celtics to advance to the in-season tournament semifinals in Las Vegas on Thursday.

"It feels good to win," Haliburton said with a smile, "especially in a game where nobody expected us to."

The Celtics, who entered the tournament with the NBA's best record, were many people's selection to win all three games this week and claim the first-ever NBA Cup. Instead, they found themselves flying back to Boston early Tuesday morning after an avalanche of plays from Haliburton and a second half left the Celtics searching for answers.

It didn't matter that Boston has the league's best defensive backcourt in Jrue Holiday and Derrick White; all Haliburton did was score or assist on Indiana's first 19 points of the third quarter, and 24 of the first 27 overall. He later buried a ridiculous four-point play on a deep 3-pointer as he was fouled by Jaylen Brown with 1:33 to go that put Indiana ahead for good -- just a couple of minutes after he juked Holiday out of his shoes with a ridiculous move and buried a 3-pointer right in front of Indiana's bench.

In total, it was a 24-minute flurry that showed the basketball world why Haliburton and the Pacers entered the night with the NBA's best offense by an absurd margin of four-points per 100 possessions - a bigger gap than the one separating the league's second-and-10th ranked offenses.

And as he was serenaded with "MVP" chants late in the second half, center Myles Turner -- who has been with the franchise since with 2015, and is its longest-tenured player -- couldn't help but soak in an atmosphere that hasn't been seen around these parts in quite some time.

"The story, the backstory of it all makes it, you know, the most special," Turner said. "And the fact that he got his opportunity, he is able to take it and run with it ... I mean, it's just dope to see where this franchise, where I started with to what it's becoming and what it potentially can become from the same level. It's honestly, like I said, it goes back to the atmosphere. I haven't heard MVP chants, maybe when (Victor) Oladipo was here, but Ty's really come in and taken over and it's fun stuff."

It was less "fun stuff" for Boston, which was left kicking itself for allowing a game to get away -- one that it held a seven-point lead in at halftime and that the Celtics had their chances to take control of in the closing minutes. But the Pacers have been a team all season long that has surprised its opponents with the hellacious offensive pace that they play with -- and the Celtics, after letting Indiana hang around when it shot 29% on 2-point shots in the first half, learned that the hard way.

"We got to just be more connected defensively," said Jayson Tatum, who finished with 32 points and 12 rebounds for Boston. "But, I mean, it's the NBA. Sometimes guys are going to make plays. He hit some tough shots. So it's kind of two fold.

"Guys get paid a lot of money to play basketball, and they're pretty good."

Haliburton certainly qualified as pretty good Monday night. Much had been made entering this game of how he had never played on TNT before this game, and the Pacers haven't been shy about expressing why this tournament was a prime opportunity to put Indiana -- and, by extension, Haliburton -- on the national stage in an entirely new way.

"The thing about the in-season tournament is that as you advance, you're going to, you're gonna play two more games, win or lose, in the quarterfinals, against better opponents," Pacers coach Rick Carlisle said. "And so this is something that's great for us because we need high level competition. It really is another thing that's a simulator of playoff basketball. So it really helps us. And so, coming out with the win tonight was big for a lot of reasons that I mentioned.

"But I know this is very special to (Tyrese), as well."

The Pacers will now savor their victory while they wait to see whether they'll face the New York Knicks or the Milwaukee Bucks in Thursday's semifinal in Sin City. But while there was a celebratory mood after Monday's win, Haliburton was insistent that the goal is still far from being accomplished.

"We've wanted to be in this situation all year, and here we are," Haliburton said.

"Now it's not just about being here. It's about winning."

Tatum and the Celtics, meanwhile, were left with a few days to ruminate on what could have been.

"Yeah. I wanted to f---ing go to Vegas," Tatum said with a rueful smile. "I didn't want to go home.

"I wanted to go Vegas, so yeah, I'm mad. Next year, I guess."