Indianapolis ? For the first 3 1/2 quarters, Paul Pierce was off-target and the Boston Celtics appeared overmatched.

By the end, Pierce had changed everything. The former Kansas University standout scored 21 of his 40 points in the final quarter, leading Boston back from a 13-point deficit as the Celtics rallied past Indiana, 103-100, in Game 1 of their first-round series.

“I thought we did a good job of putting me in position to drive,” said Pierce. “I was able to get to the basket every time. I knew I could beat my man.”

Pierce’s primary defender was Ron Artest before he fouled out with 40 seconds left. That left the Pacers scrambling to contain Pierce, whose three-pointer with 27.5 seconds left put the Celtics ahead 99-97.

Pierce went 21-for-21 from the foul line, setting an NBA playoff record for most free throws made without a miss.

Pierce blamed his slow start on illness. Bothered by a sore throat and fatigue, he missed 15 of 19 shots in the first three quarters.

Boston forward Paul Pierce, right, puts up a shot while guarded by Indiana forward Al Harrington. Pierce scored 21 of his 40 points in the fourth quarter during Saturday's first-round playoff game at Indianapolis. Boston won, 103-100.

“My legs were weak. I didn’t have any spring in my shots,” Pierce said.

“I just stuck with it. I said, ‘At least I can make some free throws and rebound.”‘

Pierce finished with 11 rebounds. Antoine Walker scored 22 points for Boston.

Game 2 of the best-of-seven series is Monday night.