A game-high 29 points from LeBron James was not enough to prevent Boston from securing dramatic late win.
NBA: Celtics tie Eastern Conference finals 2-2 with overtime win over Miami
Boston Celtics risked losing their large lead before finally seeing off Miami Heat with an overtime 93-91 victory to level the NBA Eastern Conference Finals series at 2-2.
The Celtics had built up an 18 points late in the first half, before Miami clawed themselves back into contention through aggressive play by LeBron James and Dwyane Wade.
A three-point shot by James in the final minute of regulation pushed the contest into overtime - but the league Most Valuable Player then fouled out of the game with two minutes left in the extra session, with Paul Pierce having already fouled out at the start of overtime.
Pierce scored 23 points and Rajon Rondo added 15 points and 15 assists, Kevin Garnett added 17 points and 14 rebounds for the Celtics while Ray Allen added 16 points.
LeBron James led Miami with a game-high 29 points while Dwyane Wade added 20 points and Udonis Haslem had 12 points and 17 rebounds.
After finishing regulation time level at 89-89, Haslem's slam dunk gave Miami a 91-89 lead in overtime.
But Boston's Marquis Daniels hit 1-of-2 free throws and Rondo sank a jumper to put the Celtics on top 92-91.
Rondo also made 1-of-2 from the free throw line with 21.4 seconds to play to give Boston a 93-91 edge with Miami taking the ball.
Wade elected to try a 3-point shot for the victory and evaded Daniels to create an open opportunity beyond the arc, but Wade's shot clanked off the rim and the final buzzer sounded.
"We knew they were going to Wade," Rondo said. "We played good defence on him and got the stop.
"We executed offensively, came up with some lucky plays and got some stops at the end."
Pierce tied the game at 83-83 with just over four minutes to play in regulation on an inside jumper and Rondo followed with a layup a minute later to give Boston an 85-83 edge.
After James went 1-of-2 from the free throw line, Garnett sank a jumper to boost the Celtics' lead, but Mario Chalmers answered with a driving reverse layup to pull Miami within 87-86 with 1:01 remaining.
Garnett sank an inside shot with 52 seconds to play for an 89-86 Boston lead but James answered with a 3-pointer 15 seconds later to pull Miami level again.
Garnett was whistled for a foul for knocking James to the floor with 21 seconds remaining as Rondo was racing to the basket with the ball, a call that gave Miami the ball with a chance for the last shot of the fourth quarter.
But Boston's defence stiffened and Haslem missed a shot at the buzzer, sending the game into overtime.
The Celtics, who led by as many as 18 points in the first half and 16 in the third quarter, went cold shooting from the field and saw the Heat take a 76-74 lead with a 15-1 run that was capped by a layup from rookie Norris Cole.
The Celtics struck back when Allen sank his fourth 3-pointer of the game to put Boston back in front and Garnett followed with a tip-in.
But after Wade jammed in a dunk with 5:10 remaining, the game was deadlocked at 81-81, setting up the late drama in regulation time.
The best-of-seven Eastern Conference final showdown will continue with game five on Tuesday at Miami and game six back in Boston on Thursday.
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