Boston's hopes of pipping Cleveland for top spot in the Eastern Conference suffered a blow as the Celtics slumped to defeat against the Atlanta Hawks.
The Celtics, comprehensively beaten by the Cavaliers on Wednesday, slid to another costly defeat as Paul Millsap inspired Atlanta to a 123-116 win at Philips Arena in Georgia.
Millsap scored 26 points and 12 rebounds as Atlanta improved to 40-38, a half-game ahead of the Milwaukee Bucks in the battle for fifth place.
But the defeat put a serious dent in Boston's chances of overhauling the Cavaliers at the top of the table.
The Celtics are now 50-29, lying in second spot behind reigning NBA Finals champions Cleveland who are 51-27 and now primed to lock up first place.
Millsap's points and rebounds total marked a 19th doube-double of the season, and confirmed his successful return from a left knee problem.
Millsap also got backing from Tim Hardaway Jr with 23 points as well as a career-high 20 points from Taurean Prince and 18 from Dennis Schroder.
Boston star Isaiah Thomas led the game scoring with 35 points while Jae Crowder had 24.
Elsewhere Thursday, the Washington Wizards claimed a 106-103 win over the New York Knicks to tie for third spot in the East.
Washington point guard John Wall warned however that the Wizards would need to improve their defense if they were to prosper in the playoffs.
"Our defense is terrible right now," Wall said. "If we think we're going to go to the playoffs with this defense, we'll have an early exit.
"We have to figure that out," he added after a win which lifted Washington to 48-31, level with Toronto, who had a rest day on Thursday.
- Bulls on the charge -
Meanwhile, Nikola Mirotic scored 22 points and Jimmy Butler had 19 points as the Chicago Bulls beat the Philadelphia 76ers 102-90 to boost their playoff prospects.
Butler also produced 10 rebounds and 10 assists for only the fourth triple-double of his career in what was a fifth win in six games for the Bulls.
The Bulls are locked in a neck-and-neck race with Indiana for seventh place in the Eastern Conference, with identical records of 39-40 following the Pacers' win on Thursday.
The Miami Heat (38-40) are also in the hunt, just behind Indiana and Chicago.
The Pacers moved to 39-40 after a 104-89 win over the Milwaukee Bucks.
Paul George had 23 points and 10 rebounds to subdue the Bucks, who fell to 40-39.
Indiana coach Nate McMillan said he was encouraged by the grit shown by his team on their road trip.
"We now know exactly what we have to do on this upcoming road trip," McMillan said.
"We were solid on defense tonight, and we did a lot better job attacking. Those are things we have to do. We also did a better job with our spacing tonight."
Bucks coach Jason Kidd bemoaned his team's carelessness in possession.
"We had way too many turnovers," Kidd said.
"We talked about it before the game, that we've got to take care of the ball. If you turn the ball over against Indiana, it puts you in a bad situation defensively, because they are going to take advantage of it, and they did. Once we started turning it over, we were a little hesitant to pass it."