BOSTON -- Kristaps Porzingis wasn't going to let a 37-day layoff slow him down during the biggest game of his NBA career.

Playing in the NBA Finals for the first time, the Celtics big man returned from a 10-game absence to record 20 points off the bench and Boston snuffed a second-half rally to beat the Dallas Mavericks 107-89 on Thursday night in Game 1.

Porzingis hadn't played since April 29 due to a right calf strain that he sustained during Game 4 of the Celtics' first-round series against the Miami Heat. He quickly found his groove again, though, making 8 of 13 field-goal attempts while collecting six rebounds and three blocks in 21 minutes.

"Even if I have time off, I can jump right back in and I feel the same way," Porzingis said. "I get to my spots ... so whether it's playoffs, regular season or whatever, I know how to do this. That's it, just having that confidence, going out there whatever, first round or Finals, just going out there with full confidence and giving what I have to the team."

Porzingis' early efforts helped the Celtics lead by as much as 29 points late in the first half, but Dallas got back into the game thanks to a third-quarter outburst headlined by Luka Doncic.

The star guard accounted for 10 points during the Mavericks' 22-9 surge to open the frame, with his 3-pointer cutting Dallas' deficit to 72-64 with 4:28 to go.

The Celtics responded emphatically, rattling off the next 14 points to take a 22-point lead.

Daniel Gafford closed the third with a pair of free throws to get Dallas within 20, but the Mavericks trailed by at least 17 for the entirety of the fourth.

Even though the Mavericks weren't able to recover completely, Dallas coach Jason Kidd was still able to take some positives out of the comeback bid.

"A lot of good things in that third and fourth that we can build on," said Kidd, whose team outscored Boston in both of those periods. "That's what we talked about after the game, and that's what we have to do.

"We came out and won the third. Had a great opportunity to cut into that lead. Unfortunately, they go on a run. Once we cut it to eight, but then you know we won the fourth. So there are two positive things that we can take from these four quarters."

Game 2 is set for Sunday in Boston.

Jaylen Brown led the Celtics with 22 points and added six boards, three steals and three blocks. Jayson Tatum chipped in 16 points and 11 rebounds, while Derrick White netted 15 points.

Thursday's victory gave Tatum flashbacks of the 2022 Finals, in which Boston downed the Golden State Warriors in Game 1 before going on to drop the series in six games. Because of that, Tatum is focused on making sure the Celtics don't ease up on Sunday.

"It definitely does feel good to win the first game," Tatum said, "but we know that two years ago we won the first game and the outcome of that series. So we still have a lot of work to do."

Doncic recorded 30 points and 10 rebounds for Dallas, which was outshot 47.6 percent to 41.7 percent overall. P.J. Washington went for 14 points, Jaden Hardy scored 13 and Kyrie Irving, showered with boos all night, had 12.

"I thought it was going to be a little louder in here, but I'm expecting the same things going into Game 2," said Irving, who signed with the Brooklyn Nets instead of re-signing with the Celtics in June 2019. "Crowd trying to get me out of my element, my teammates out of my element."

Although Porzingis didn't draw the start, he wasted no time setting the tone after checking in with 7:17 left in the first quarter.

The big man tortured his former team, scoring 11 points and blocking a pair of shots in the period to lift the Celtics to a 37-20 advantage. No team has ever built a larger lead in the first quarter of Game 1 in Finals history.

Boston kept pouring it on, using a 14-2 run to go up 58-29 with 4:11 remaining in the second quarter. However, Doncic erupted for nine points over the final four minutes of the first half, helping the Mavericks pull within 63-42 by the break.

--Nick Galle, Field Level Media