The Celtics will play the Philadelphia 76ers, who eliminated East No. 1 seed Chicago in six games with a 79-78 victory earlier Thursday.
Garnett topped his regular-season high of 25 points and had five blocks and three steals for Boston, which has won at least one playoff series every year since the new Big 3 was assembled. Paul Pierce had 18 points despite playing with a sprained medial collateral ligament in his left knee, and Rajon Rondo had 14 points and eight assists.
Josh Smith had 19 points and nine rebounds for Atlanta, which failed to advance in the playoffs for the first time in four years. Joe Johnson had 17 points, Marvin Williams added 16 and eight rebounds and Al Horford had 15 points and nine rebounds.
But Horford missed the first of two foul shots with 2.3 seconds left after he was tackled by Marquis Daniels to avoid an easy dunk that would have tied the game. After he made the second, the Celtics got the ball to Pierce, he was fouled and made both free throws.
The Celtics will open the second round against Philadelphia in Boston on Saturday night. The crowd burst into a cheer – even the Celtics players were checking it out – near the end of the third quarter when the final play of the Sixers-Bulls game was shown on the scoreboard.
Atlanta led 79-78 when Garnett made a turnaround jumper in the lane over Smith to put Boston ahead by one. Smith missed a long rainbow jumper and Ray Allen got the rebound with 10 seconds left, but he made only one of two free throws.
With 10 seconds left, Johnson drove to the basket but Pierce blocked his shot and Garnett tipped it out of bounds with 3.1 seconds left. The Hawks got the ball in to Horford in the lane but as he went up for the dunk or layup Daniels, who wrapped him up and kept him from scoring.
The first foul shot went off the rim, costing Atlanta a chance to tie it.