Pierce hit a fade-away jumper to seal the deal with 0.5 seconds remaining in Wednesday night's game to push the Celtics an 8-1 record in the rematch of last year's first round of the playoffs.
The Celtics trailed at one point by 16, but fought back to tie the game at the half. It was a nail-biter throughout. Pierce led the way with 34 points, Garnett added 25 points and 12 rebounds, and Ray Allen contributed 17 points.
Marvin Williams hit a go-ahead three with seven seconds remaining, before Pierce's game-ending shot hit the nylon.
Joe Johnson scored 28 points for The Hawks.
Atlanta came into TD Banknorth Garden undefeated—before the defending champion Celtics put them in their place, proving they still are the team to beat in the NBA.