INDIANAPOLIS, April 19 (Ticker) -- Paul Pierce used a foul shooting clinic to carry the Boston Celtics to an unlikely victory.

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Pierce drained all 21 free-throw attempts and poured in 21 of his 40 points in the fourth quarter as the Celtics rallied from a 16-point deficit for a stunning 103-100 victory over the Indiana Pacers in the opener of their Eastern Conference first-round series.

Pierce, who also grabbed 11 rebounds, presented a huge matchup problem for Indiana all game but never moreso than in the fourth quarter. He repeatedly used his superior perimeter skills to draw fouls from the Pacers on forays to the basket, making all 11 free throws in the period.

"It became pretty obvious that Paul could get to the line, so we went with what worked," Celtics coach Jim O'Brien said.

Indiana took its final lead at 97-96 on two foul shots by Brad Miller with 32 seconds left. That set the stage for Pierce, who nailed a step-back 3-pointer over the arm of Jeff Foster five seconds later to put the Celtics ahead for good.

It was a far cry from his struggles from the floor, where Pierce was just 4-of-19 through three quarters and finished 8-of-24.

"I was pretty much tired coming into the game," Pierce said. "I really didn't feel well. I had a sore throat. Early in the game, my legs weren't there and I couldn't hit my jump shot. But the least thing I could do was hit some free throws."

Pierce's 21-for-21 effort set a playoff record for most free throws without a miss.

The Pacers' Al Harrington missed a 3-pointer with nine seconds left and Pierce made a pair from the line for a four-point lead. But Jamaal Tinsley gave Indiana hope by nailing a 3-pointer with three seconds to go - just the Pacers' second basket over the final 8:19.

Pierce made it 103-100 with two free throws with 2.6 seconds left. Indiana got a half-court shot from Jonathan Bender that was well short as time expired.

"We're down, but it's a long series," Tinsley said. "We can't dwell on it, it's just one game."

Ron Artest led the Pacers with 26 points before fouling out. Jermaine O'Neal added 24 but scored just two in the fourth quarter as the Celtics' swarming defense forced the ball out of his hands.

"We really just started missing shots down the stretch," Pacers coach Isiah Thomas said. "We got downcourt and we couldn't handle the basketball. It was a tough stretch of game for us."

Indiana led for almost the entire second half and grabbed its largest advantage, 79-63, on a basket by O'Neal with 3 1/2 minutes left in the third quarter. The lead was 88-75 after O'Neal's free throw with 7:42 remaining.

"Our goal was to get it to 10, and that is what we had to do going into the fourth quarter," Pierce said. "Once we got it to 10, we tightened up our defense and we started to get a lot of easier looks."

Pierce responded with a three-point play, Tony Delk nailed a 3-pointer and Antoine Walker scored on a driving layup to quickly slice the deficit to five with just over six minutes remaining. Walker scored 10 of his 22 points in the fourth quarter.

"It's a big win," Walker said. "We stepped up, stayed confident and made big plays when we needed."

A running jumper by Miller gave the Pacers a 93-85 cushion with 3:35 left to play before Pierce scored seven points in a 9-2 tear that gave Boston its first lead since the first quarter, 96-95.

The final two points came were free throws by Pierce, who grabbed a defensive rebound and drew a foolish foul on Artest, his sixth.

"It wasn't a foul," Artest said. "It was a jump ball, but they saw it a different way."

The Pacers played most of the fourth quarter without veteran shooting guard Reggie Miller, who played just two of the final 12 minutes. He failed to score in the fourth and finished with 13 points.

"We needed one stop and we put our best defenders out there," Thomas said. "We just didn't get that stop."

Boston also overcame the ejection of starting center Tony Battie for a flagrant foul in the second quarter. Battie played just 10 minutes and had four points and four rebounds.

The Celtics got support for Pierce and Walker from Eric Williams, who scored 18 points on 7-of-12 shooting. Delk chipped in 12 points.