Evan Turner scored the go-ahead layup and drained a couple of key free throws down the stretch as Philadelphia held on for a 82-81 win over Boston to even their Eastern Conference playoff series at 1-1.
Turner drove to the basket, got knocked down but still made the layup as he was falling under the basket to give the upstart Sixers a 76-75 lead 40 seconds left in the fourth.
It put Philadelphia up to stay and came after a flurry of threes by both teams that had the score seesawing back and forth.
"You play a game like this," said Sixers coach Doug Collins, "and it's worth like 10 regular season games in terms of experience."
Jrue Holiday scored 18 points, Andre Iguodala had 13 for Philadelphia who became just the fifth No. 8 seed to win a first-round series when they knocked off the Chicago Bulls.
Rajon Rondo finished with 13 assists, eight points and seven rebounds for the Celtics.
Philadelphia outplayed Boston for most of game one until the Celtics' experience took over down the stretch in a 92-91 Boston victory.
The Sixers nearly squandered another double-digit lead but managed to hold on down the stretch thanks to a pair of free throws by Turner with 12 seconds left that made it 78-75.
Boston's Kevin Garnett hit a three pointer at the buzzer for the one-point margin. He was called for an offensive foul on a key Celtic possession in the final seconds that led to two more free throws for the Sixers which they made.
"I wasn't fond of it," said Boston coach Doc Rivers, who said the officials were letting similar screens go all night. "But we put ourselves in that position ... to let someone else do something (to win the game)."
Game three of the series will take place Wednesday in Philadelphia.
Philadelphia took two of three games from the Celtics during the regular season.
The Celtics defeated the Atlanta Hawks in six games to advance to the second round for the fifth consecutive year.