ATLANTA — Welcome back, Al Horford.

In his first start since January, Horford scored 19 points, hit Atlanta’s final two baskets and came up with a huge defensive play to make sure the Hawks kept their season going with a frenetic 87-86 victory against the Boston Celtics in Game 5 of the Eastern Conference playoffs Tuesday night.

“I wanted to bring that energy to the team,” Horford said.

That he did, dribbling out at the head of fast breaks and diving fearlessly on the court for loose balls, not worried about reinjuring the pectoral muscle that kept him out most of the season. In addition to leading the Hawks in scoring, he grabbed 11 rebounds, dished out three assists, came up with three steals and blocked three shots, all while playing more than 31 minutes.

The Celtics lead the series 3-2 heading back to Boston for Game 6 on Thursday night. If the Hawks can steal one on the road, the deciding game would be Saturday in Atlanta.

“We can be nothing but confident,” Horford said. “We know it’s going to be a big challenge up there. “

Boston had a chance to clinch the series when Rajon Rondo stole Josh Smith’s inbounds pass with 10 seconds remaining and raced down the court, looking for the winner. But Rondo got hemmed in along the sideline when Horford jumped out to defend him, which caused Rondo to lose control and fling a desperation pass. Smith knocked it away, the ball rolling out of bounds as the horn sounded.

“I cornered myself,” Rondo said. “Give Al credit.”

A relieved Smith collapsed on the scorer’s table as glittery streamers fell from the rafters.

“I feel awful the game ended the way it ended because I thought Rondo willed us back into the game,” Celtics Coach Doc Rivers said.

Paul Pierce and Kevin Garnett led the Celtics with 16 points apiece. Rondo had 13 points and 12 assists, and the last of his five steals gave Boston a chance to end the series early and get some much-needed rest.

“This is the team they thought they were going to start the season with,” Rivers said. “They now have it and we have to deal with it.”

In a back-and-forth game, the Celtics made it 83-83 on Ray Allen’s 3-pointer from the corner, capping a quick 7-0 run.

The Hawks called time and set up a play that reclaimed the lead for good. Jeff Teague drove into the lane and dumped it off to Horford for a dunk.

After Garnett missed a jumper in the lane, Joe Johnson ran down the shot clock looking as though he had every intention of shooting. But at the last possible second, he rifled a pass to Horford coming off the wing. Horford threw up a sort of pseudo-hook with Garnett and Brandon Bass all over him. The ball bounced around, then in, giving the Hawks an 87-83 lead.

With less than a minute to go, after misses by Pierce and Horford, Pierce was left open for a 3-pointer. He swished it, pulling the Celtics within a point. They wouldn’t get another, even with a couple of chances.

Teague missed an open 3 for the Hawks, giving Pierce a look at the go-ahead shot. But his 18-foot jumper with Johnson in his face barely touched the bottom of the net while Pierce looked around in disbelief, believing he must have been fouled.

Rondo’s steal gave the Celtics one more chance, but they never got a shot off.