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.
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.
"It wasn't a foul," insisted James, who also voiced his displeasure after the game with a couple of other calls and not getting a timeout granted.
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.
"Everything went wrong for us in that fourth quarter, I mean every single thing," said Cavs coach Tyronn Lue.
Even Hawks coach Mike Budenholzer noted: "A fourth quarter like that is hard to explain".
For James, the explanation was simple.
"At the start of the fourth, we didn't get stops," he said. "When you let a team stick around, this is what happens.
"Obviously this has been a very challenging year for us, but we've got two more games and then get ready (for the playoffs). We've had some good moments and some not-so-good moments -- like today."
- Huge win for Hawks -
James said he wasn't concerned with the possibility of going into the post-season seeded behind Boston.
"For me, it's never been about trying to get the number one seed," said the four-time NBA Most Valuable Player, who instead wants to see the Cavs playing good basketball heading into the playoffs.
That certainly wasn't the case in the fourth quarter on Sunday.
"That should never happen," Lue said of the collapse. "Especially when we are such a great offensive team, to be out-scored by 26 points in a quarter."
In fact, it was just the third time in NBA history that a team overcame a deficit of 26 or more points in the fourth quarter and won.
"Any time you beat the defending NBA champions it's huge," Hardaway said, "no matter the circumstances."
The win gives Atlanta a one-game lead over the Milwaukee Bucks for the fifth seed in the Eastern Conference.