The Celtics locked up the No. 1 seed in the Eastern Conference late on Sunday night, and a night later, it looked like they had no interest in slowing down despite that security. They went up by 30 points on the Hawks in the first half, and a 10th consecutive victory seemed inevitable.

But then a group that has rarely been exposed this season seemed to finally run into complacency.

The Celtics got lazy, they relaxed and maybe even got bored. They played with the game, and it came back to haunt them. A poor end to the first half carried into a disastrous second half, and the Celtics suffered their most humbling loss of the season in a 120-118 defeat to the Hawks on Monday night in Atlanta.

Jayson Tatum scored 37 points and Jaylen Brown added 24 points for the shorthanded Celtics, who were missing injured guards Derrick White and Jrue Holiday. But there was no excuse for this loss.

The Celtics let their foot off the gas defensively, they lost their physicality, and suddenly pushed into a tight game, they couldn’t get the job done.

Despite a poor second half, the Celtics led by two with less than two minutes to go. But Tatum committed a turnover, which led to Bogdan Bogdanovic’s go-ahead transition 3. Brown responded with an elbow jumper before Dejounte Murray answered by beating Al Horford for a layup.

The Celtics, trailing by one, then had a poorly executed offensive possession that resulted in a low-quality fadeaway 3-pointer from Brown that missed. On the next possession, Murray missed a jumper but Clint Capela came up with the offensive rebound before finding De’Andre Hunter, who drilled a 3-pointer above the break with 10 seconds remaining that put the Hawks up four and clinched their comeback victory.

The Celtics led 68-38 after an Al Horford 3-pointer with 4:23 remaining in the second quarter after a scorching hot shooting start before it was the Hawks’ turn. Murray, Bogdanovic and Hunter led the Hawks – who were without injured star Trae Young – on a 37-10 run that suddenly cut the Celtics’ seemingly insurmountable lead to one possession.

The Celtics punched back but lost their lead in the fourth quarter when Wesley Matthews’ 3-pointer put the Hawks up by one. And they got uncharacteristically sloppy and disengaged. Tatum was whistled for a rare eight-second violation, and even Payton Pritchard missed a pair of free throws. Bruno Fernando’s putback layup put the Hawks up four midway through the fourth, and the Celtics were almost shocked.