The Atlanta Hawks seized their chance as the Cleveland Cavaliers collapsed late, erasing a 26-point second-half deficit to stun the NBA champions 126-125 in overtime on Sunday.
The Hawks, who were short-handed when they shocked the Cavs in Cleveland on Friday, never led until overtime.
They took the lead for good on a three-pointer by Mike Muscala with 35 seconds remaining.
Kent Bazemore had a steal and basket with 22 seconds left, and Tim Hardaway Jr. made two clinching free throws with 3.6 seconds to play.
Kyrie Irving capped a 45-point game for Cleveland with a basket at the buzzer that made no difference.
Superstar LeBron James had a triple-double of 32 points, 16 rebounds and 10 assists but fouled out in overtime.
The Cavs led 93-67 after three quarters. But Atlanta out-scored them 44-18 in the fourth.
Paul Millsap's jump shot at the buzzer to end regulation knotted the score at 111-111.
The defeat dropped Cleveland, at 51-29, into a tie with the Boston Celtics for the top seed in the Eastern Conference, although the Cavaliers own the tiebreaker.
Both teams have two regular-season games remaining.
The Hawks were without five key players when they beat the Cavs 114-100 in Cleveland on Friday.
Although they had all but one of those absentees back on Sunday, the Cavaliers appeared to be rolling toward a payback victory until the fourth quarter.
Cleveland hit their first three three-pointers and had seven from beyond the arc as they took a 38-21 lead after one quarter.
Cleveland led 65-46 at halftime.
But it unravelled for them in the fourth, as Atlanta scored the first nine points of the quarter to launch their comeback.
The win gives Atlanta a one-game lead over the Milwaukee Bucks for the fifth seed in the Eastern Conference.