The Celtics led for the final 46 minutes 40 seconds of Sunday’s game against the Sacramento Kings. They scored 128 points, shot 56 percent from the field, and won the rebounding battle by 17. It was an efficient follow up to Friday’s stunning victory over the Cleveland Cavaliers.

Yet coach Brad Stevens and the players were rather blah about their 128-119 victory. They nearly blew a 21-point lead and their defense was rather suspect two days after standing strong against the best team in the Eastern Conference.

The Celtics’ rather uncomfortable sentiment after a win shows the progress the team has made the past month. They have won nine of 10 games since a Jan. 20 loss at Toronto dropped them to 22-21, and with Atlanta’s loss Sunday have the third seed in the Eastern Conference to themselves.

It’s the first time since April 26, 2012 — the Doc Rivers era — that the Celtics are at least nine games over .500.

They are approaching the All-Star break with the type of momentum they desperately wanted. But this game was a struggle. The Celtics scored 74 points in the first half and looked as if they would coast to the win before the Kings sliced the lead to 5 late in the game on a DeMarcus Cousins 3-pointer.

On his 27th birthday, All-Star Isaiah Thomas secured the victory by scoring 9 of the Celtics’ final 13 points against his former team. The usually defensive-minded Celtics were lured into a shootout and barely had enough firepower to prevail.

“After the game we had the other night, we didn’t want to let ourselves down by not playing the right way,’’ Thomas said. “We came out and played the right way in the first half. They made a run, which we knew they were going to make, but we sustained it.’’

Facing the team that allows the most points in the league, the Celtics set a season high in scoring. The 74-point first half was the Celtics’ biggest since Jan. 25, 2009, against Dallas, and their 46-point first quarter was their best opening period since Feb. 12, 1982, against the San Diego Clippers.

The Celtics shot 58.1 percent and hit seven 3-pointers in the opening period. Avery Bradley scored 12 of his 27 points in a 6:20 stretch of the quarter. But Boston yielded 37 points to Sacramento, giving Stevens a reason to be concerned.

The Celtics committed a season-high 24 turnovers in all, becoming careless as the game progressed.

“If you look at the [Dec. 3 game against the Kings] in Mexico City, it was similar,’’ Stevens said. “We had our runs when we played really well. We were throwing it to our team, which is usually a good idea. We were making shots and you get out to that lead. And then the game gets sloppy. We were talking about trying to avoid that.’’

The Kings were incapable of getting key stops when it counted, and the Celtics finished with three 20-plus-point scorers — Bradley (25), Thomas (22), and Jared Sullinger (21).

Cousins, rumored for two years as a Celtics trade target, led all scorers with 31 points, including 14 in the fourth quarter. But he showed some of the faults — technical foul, shabby defense — that have made him a polarizing figure in the NBA.

Former Celtic Rajon Rondo returned to TD Garden for the second time since Boston traded him to Dallas on Dec. 18, 2014, and he totaled 14 points, 15 assists, 5 rebounds, and 6 steals in 39 minutes.

The Celtics are 62-50 since dealing Rondo, who acknowledged the team’s improvement and was quite complimentary of Thomas.

“Obviously he’s an All-Star on the team, the plays he’s able to make out there, especially with his height and his heart, he’s been unstoppable this year,’’ Rondo said. “He played well against us [in Mexico City]. He hit a big three [Sunday] that pretty much sealed the game. It was a big, tough shot he made. I think that’s what he’s been doing all season for these guys.’’

Tyler Zeller scored a season-high 17 points in 21 minutes along with 7 rebounds and 2 blocked shots while taking on Cousins. He had a 3-point play as well as two free throws during a Celtic mini-spurt in the fourth quarter.

Zeller has delivered when called upon of late, averaging 13,7 points and 6.5 rebounds and shooting 68.6 percent in the last four games.

“One thing about Tyler: Tyler’s always going to run the floor really hard and he’s always going to roll to the rim really hard,’’ Stevens said. “And so you’ve got a stable presence at the rim, so you know that, and we were getting some good things from our shooters, and so to have a guy that’s going to be at the rim is going to open up some opportunities for the guys on the perimeter and it’s going to hopefully end up opening opportunities for him.

“I thought he did a good job obviously in playing in the seams and knocking down a few open shots, too. So Tyler’s doing great.’’

Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.