BOSTON – Kristaps Porzingis did everything he could over the past five-plus weeks to prepare his body in time for the NBA Finals. On Thursday night, his diligence paid off.

Boston’s 7-foot-3 center stepped back on the court for the first time since straining his calf on April 29 and played as if he’d never left. He opened the Finals by torching the Dallas Mavericks with 20 points in just over 20 minutes of action, leading the Celtics past his former team for a 107-89 Game 1 win.

In the days leading up to his return, Porzingis admitted he had no idea how he’d feel until he put his feet back on the parquet floor. As soon as he walked out to the roaring crowd at TD Garden, his confidence rushed back and he felt at home.

“The adrenaline was just pumping through my veins,” he said. And it carried him to 20-point, six-rebound, three-block performance.

For the first time in more than seven years and just the second time in his career, Porzingis came off the bench. But it didn’t matter to him from either a personal or a performance standpoint whether he was starting or playing in a reserve role.

“In this situation, it made sense,” Porzingis said. “I didn’t care. I knew I could prepare to come off the bench, which is something different for me, and that’s what I did and stepped into that role and embraced it and had a good game.”

Porzingis subbed in for Al Horford at the 7:17 mark of the first quarter and proceeded to go off. Throughout the remainder of the frame, he logged 11 points on 4-of-5 shooting, three rebounds, and two blocks.

“He was just making play after play,” Jaylen Brown recalled of Porzingis’ initial stretch. “It was just like, ‘Alright, he’s back.’ There was no question about it.”

KP finished the half with a game-high 18 points on 7-of-9 shooting in just under 13 minutes of action, leading Boston into the break with a 63-42 lead. Jayson Tatum noted after the game how that half showcased how impressive Boston can be when it’s fully healthy.

“We’re a good team when guys are down, but we’re really, really special when we have everybody,” Tatum said. “How seamless his transition was after being away for six weeks, and coming into the game and having a first half like that, that was a big spark for us to start the game.”

As much as Dallas’ defenders got in his face, they just couldn’t stop KP from putting the ball in the basket. He finished the game 8-of-13 from the field, 2-of-4 from 3-point range, and 2-of-2 from the free-throw line.

“He’s a matchup nightmare,” said Derrick White. “Even when you play good defense, he doesn’t really see you. It’s been unreal just watching him all year. Then defensively, he uses his size well, just impacts the games in so many different ways. He changes us and he makes us a better team.”

Porzingis’ presence opened the floor for the whole team. All six Celtics who topped 20 minutes scored in double digits, led by Jaylen Brown’s 22-point effort.

“He was great on both ends of the floor, defensive execution, game plan, playing for a spot in the offensive end, being physical, and making plays on both ends of the floor,” said coach Joe Mazzulla. “He played great and that’s the KP that helped us get to where we are today.”