"Pierce exploded and we just couldn't get him under control," Pacers Coach Isiah Thomas said.
The loss was especially bitter for the Pacers, who squandered a 16-point, third-quarter lead and were pushed to the brink of elimination.
After the Pacers built their biggest lead at 52-36 early in the third quarter, Pierce took matters into his own hands, more than doubling Indiana's total the rest of the quarter with 21 points as the Celtics outscored Indiana, 37-10.
"I knew I was going to have to be more aggressive in the second half," Pierce said. "I was really passive (in the first half) because I tried to get others involved. That's the way we won Game 3. But when my opportunities are there, I'm going to take advantage of it."
Pierce started his assault with a three-pointer with 9:33 left in the third quarter and added a three-point play and a 19-foot jumper as the Celtics went on a 15-0 run and pulled within 52-51 with 7:11 left in the period.
Boston took its first lead of the game at 59-58 with 4:50 remaining on a layup by Antoine Walker, who followed that with a three-pointer.
Thomas called a timeout to settle his team and it initially worked as Jermaine O'Neal and Harrington scored inside to tie it at 62-62 with 3:14 left.
But Pierce scored the last 11 points of the quarter for a 73-62 advantage. He started the surge with a three-pointer over Artest with 2:28 left and hit a 20-footer as he was fouled by Artest 22 seconds later, completing the three-point play and giving the Celtics a 68-62 lead.
"That's Paul Pierce, period," said Celtics forward Eric Williams. "It's just one of those things where he got hot, hit a couple of threes, got to the basket, made a couple of passes and he just had it all going."
Pierce pulled up for a foul line jumper with 1:32 remaining, then capped his third-quarter performance with a three-pointer over Harrington with 3.5 seconds left.
"I got on a roll and they just kept giving me the ball," Pierce said. "I hit two or three in a row and just fed off the crowd. My adrenaline was going and that's what sparked it."
"I'm just glad he's on our side. He took over when we needed him to," Boston forward Walter McCarty said.
Pierce was talking to Harrington as he was dribbling in the final seconds of the quarter before stepping back and hitting a 25-foot jumper as the FleetCenter crowd exploded.
"He was just saying `I hope you're ready,'" Harrington said. "I was saying `I'm ready, bring it.' I was hoping he drove. He took the step-back shot and I just couldn't get to it."
The Pacers hit just four of 16 shots in the quarter while Pierce was seven of 10 from the field.
"We started getting down and dirty and played defense," Walker said. "We said to ourselves that we were going to be the aggressors in the second half, get in their face. If you look at how they were scoring early, they were getting layups and we weren't contesting shots."
The Pacers never really recovered in the fourth quarter, although they closed within 81-76 on a driving layup by Brad Miller with 5:35 left.
Walker took advantage of a mismatch against Ron Mercer, hitting an 11-foot jumper and a hook shot in the lane to give the Celtics an 85-76 advantage with 3:49 left.
A three-pointer by Tony Delk, who finished with 21 points, increased the lead to 90-78 with 2:04 remaining.
Pierce, who also had 10 rebounds and seven assists, hit a fadeaway jumper and five free throws in the final 84 seconds to seal the win. He was 10 of 21 from the field and made 14 of 15 free throws.
The 32 points in the second half eclipsed the club playoff record of 30, shared by Larry Bird and John Havlicek.
"(Pierce) didn't have very many open looks," Thomas said. "There was a hand in his face and every shot was challenged. He is a shotmaker and he made shots. I don't think Artest could have been any closer to him."
Indiana's Reggie Miller, who missed all seven of his field goal attempts in Game 3, did not make his first basket until a meaningless three-pointer with 42 seconds left. He sat most of the second half and finished with three points.
O'Neal had 25 points and 19 rebounds and Brad Miller added 21 points for the Pacers, who are 0-11 when trailing a series 2-1.
"I don't even know how to explain it when we get these big leads and then we can't hold on," Brad Miller said. "We start missing shots and get outside of our offense."
The series returns to Indiana for Game 5 on Tuesday night.