ORLANDO -- The Celtics had a golden opportunity to open the Eastern Conference finals with a win, building a 14-point fourth-quarter lead.
But Game 1 against the Miami Heat slipped away from the Celtics, who were overtaken in overtime, 117-114, thanks to the heroics of Jimmy Butler and Bam Adebayo on Tuesday night.
Game 2 will be played at Disney World on Thursday at 7 p.m.
Butler made a 3-pointer with 22 seconds left in regulation and then converted a three-point play with 12 seconds remaining in overtime to open a 116-114 lead.
Adebayo then blocked a dunk attempt by Jayson Tatum with three seconds to go to prevent the Celtics from tying the game.
Tatum had 30 points, 12 rebounds, five assists, three steals and two blocked shots. He has had five straight double-doubles in the playoffs.
Marcus Smart scored 26 points, hitting six 3-pointers while Jaylen Brown scored 17 and Brad Wanamaker chipped in with 11 points, six rebounds and five steals.
Kemba Walker struggled with his shot for a third game in a row, missing 13 of 19, including 1 of 9 from 3-point range. He had 19 points and six assists.
Goran Dragic scored 29 points to lead the Heat, who are 9-1 in the playoffs. Former Celtics forward Jae Crowder made five 3-pointers and had 22 points while Butler finished with 20 points. Adebayo scored 18 and Tyler Herro had 12.
The Celtics were up, 110-106, early in OT but added just a Brown tip-in and a Walker step-back over Herro with 23.2 seconds left that gave the Celtics a 114-113 advantage.
Butler then drove on Tatum and scored, adding a foul shot for the two-point lead. After Adebayo’s block and ensuing free throw, Tatum put up a desperation 3-pointer that missed at the buzzer.
The Celtics was tied at 106 after regulation with Butler putting the Heat ahead, 106-105, when he hit a corner 3-pointer with 22 seconds to go.
Derrick Jones Jr. was then inserted into the Miami lineup for defense, but he ran over Smart before the ball was thrown inbounds, giving Tatum a free throw to tie the game.
The Celtics then ran the clock down, but a 3-pointer from the left by Tatum hit the front of the rim just before the buzzer.
The Celtics had held an 85-71 lead when Walker hit a stepback in the opening minute of the fourth quarter and they were in front, 92-82, with 8:19 left after a steal and layup by Wanamaker.
But the Heat went on an 8-2 run in less than two minutes to get within 94-90. They were behind, 101-95, after two Walker free throws with 3:36 left, but Herro hit a 3-pointer.
After Tatum set up Daniel Theis inside to give the Celtics a 103-98 lead, Adebayo hit two foul shots before Walker scored over Crowder on a drive to put the Celtics on top, 105-100, with 1:09 to go.
Herro hit another 3-pointer with 1:03 remaining and after a shot clock violation by the Celtics, Crowder made the 3-pointer to put Miami up for the only time in the quarter.
After giving up 37 points on 68 percent shooting in the second quarter, the Celtics tightened up on the defensive end in the third quarter to take an 83-71 into the final 12 minutes.
The Heat had their lowest scoring quarter of the postseason with 16 points, making just 6 of 20 shots, including 1 of 8 from 3-point range.
The Celtics finished the quarter on a 13-2 run in the last 2:20 after the Heat were within 70-69.
Tatum had six points to lead the way in that spurt, Grant Williams made a corner 3-pointer, Walker hit three foul shots and Smart made a free throw.
Earlier in the quarter, the Celtics had scored nine straight points to change a 61-56 deficit into a 65-61 lead with 6:39 remaining. Smart and Walker hit 3-pointers in the span of 39 seconds, and Smart added another.
The first half, which ended tied at 55, included a first quarter that was carried by the Celtics and a second quarter owned by the Heat.
The Celtics opened the game with a 12-3 running, getting five points each from Smart and Tatum in the first four minutes. The Heat began the game missing nine of their first 10 shots.
The only points scored by the Celtics in the final four-plus minutes were a pair of foul shots by Brown with 59 seconds to go after the Heat scored seven straight points.
Miami shot just 27 percent (6 for 22), its worst performance of the playoffs in a quarter, and was held to its lowest point total in a quarter in the postseason (until its 16-point third quarter).
Tatum opened the second quarter with a 3-pointer to push the lead back to double digits at 29-18 and the Celtics were in front, 36-26, with 8:10 to go in the half on another Tatum basket.
But the Heat started moving faster at the offensive end with Dragic (16 in the half) leading the way and outscored the Celtics, 13-2, in just under two minutes.
The Heat took their first lead on a Butler basket, making it 39-38 with 5:59 remaining.
The Celtics were behind, 52-46, after Crowder made a 3-pointer with 2:11 left in the half, but a pair of 3-pointers by Brown, the last one coming with 10.7 seconds to go, created the tie game at the half.
Tatum had 15 points, six rebounds, four assists and 2 blocks in the half while Smart added 13 points. Walker contributed just two points, missing all four of his 3-pointers and shooting 1 for 7 overall.
The Heat had their best shooting quarter of the playoffs (15 for 22, 68 percent) and made 6 of 8 shots from 3-point range.
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