Evan Turner made a running scoop shot with 0.2 seconds left to give Boston its only lead of the game, and the Celtics sent the Atlanta Hawks to just their third loss of the calendar year, 89-88 on Wednesday night.
Jared Sullinger had 17 points and 15 rebounds for the Celtics, who have won four of five but still remain out of playoff position. Turner had 12 points, nine assists and seven rebounds.
Al Horford scored 22 points and added 12 rebounds for the Hawks, who followed a 19-game winning streak by losing three of their last six. Atlanta still has the most wins in the NBA heading into this weekend's All-Star break.
Atlanta scored the first nine points of the game and nursed the lead through most of the next four quarters. The Hawks led by 14 with 7 minutes left before Boston scored the next eight points. It was 84-75 when the Celtics ran off seven in a row, including a 3-pointer from Marcus Smart with 2:12 to play that made it a two-point game.
Avery Bradley made a 3-pointer to cut an 86-82 lead to one point, then the teams traded baskets and misses.
Jeff Teague missed a runner with 15 seconds left, and the Celtics called a timeout with 6.4 seconds on the clock. Turner dribbled at the perimeter before cutting into the lane and floating an 11-footer in just before the buzzer.
The Hawks had a chance for a tip-in, but the inbounds pass was knocked harmlessly away by the defense.
It was a shocking upset to send the Hawks, who will send an NBA record-tying four players to the All-Star Game in New York, into the break. The last team to place four players in the All-Star Game was Boston in 2011; the Celtics have no All-Stars this season.