Kyle Korver helped bring the Hawks back from a 27-point hole with eight 3-pointers, and Atlanta held the Celtics to just four points in the second overtime to earn a hard-fought, 123-111 victory at Philips Arena.
Boston was well on its way to snapping a five-game losing streak, but folded after halftime in stumbling to its worst losing streak since the 2006-07 season -- the last time the club failed to win the Atlantic Division.
Korver netted 24 of his 27 points after the break, while Josh Smith and Al Horford each chipped in double-doubles in Atlanta's third straight victory.
Rajon Rondo totaled 16 points, 11 assists and 10 rebounds for his second straight triple-double in a losing cause, and Kevin Garnett provided 24 points and 10 boards before fouling out early in the second OT.
The Hawks trailed by 27 at one point in the second quarter, but tied the game with a 19-0 run in the third and went ahead, 77-75, following two Horford free throws early in the fourth.
After trading baskets, the Celtics used a 9-0 run to build another double- digit lead, but Korver ended the rally with a 3-pointer and hit two more from behind the arc on back-to-back possessions to pull within 98-96.
"Guys did a good job helping me get open and passing to the right spots," Korver said. "I don't create shots on my own...so I have to give my teammates a lot of credit."
With one minute remaining in regulation, Horford made two at the foul line to tie things. Garnett was short on a floater on the ensuing possession, and Devin Harris lost the ball out of bounds at the other end.
Rondo hit the back iron on a floater as the clock dipped under 10 seconds, and the rebound wound up on the hardwood before a jump ball between Rondo and Korver was called. Atlanta controlled the ball after the tip and called timeout with 3.7 seconds remaining. Harris threw a lob to Smith near the rim, and Paul Pierce smothered Smith into an airball as the buzzer sounded.
It was locked up at 107-107 with under a minute to play in the first extra session. Korver and Harris each missed 3-point attempts, and Rondo came away with the rebound on the latter before calling timeout with 23.6 ticks remaining.
The Celtics dribbled down the clock and it was Pierce who dribbled into the lane for a step-back jumper that Smith got a piece of to force another overtime.
Atlanta scored the first 10 points of the second OT to pull away. Smith's three-point play off a putback layup started things off, and Garnett committed his sixth foul going after an offensive rebound with 3:40 to play.
Horford scored the next five points to finish his 24-point, 13-rebound performance, and Harris essentially sealed the win on a driving layup with under 90 seconds on the clock.
Back on Jan. 5, in the first meeting of the season between these perennial Eastern Conference playoff contenders, the Celtics took control with a 33-9 third quarter that vaulted them to an 89-81 victory.
They had another one-sided quarter on Friday, this time in the opening frame as they kept the Hawks off the scoreboard over the final five minutes en route to a 29-10 advantage.
The lead grew to as high as 27 in the second, and it was a 57-38 game at halftime.
Atlanta, after shooting 31 percent from the floor and allowing Boston to hit 53 percent of its shots in the first half, turned things around on both ends in the third quarter. Boston went more than six minutes without scoring during a 19-0 flurry that tied things at 59-all. Korver drained two from behind the arc during the run, which was capped by Smith's dunk off a turnover.
"When we get a lead...we start acting like we're the 2008 (championship) team. We haven't earned that right," Celtics head coach Doc Rivers said.
Buckets by Rondo and Pierce in the final minute of the third gave Boston a 73-69 edge.
Pierce shot just 4-of-13 from the floor for 12 points ... Smith totaled 17 points, 14 rebounds and seven assists, while Jeff Teague chipped in 23 points for the Hawks, who improved to 4-0 in overtime games this season ... The Celtics, who host the Miami Heat on Sunday, committed 21 turnovers.