Brad Stevens felt improved defense helped the Boston Celtics turn Game 5 against the Miami Heat to stay alive in the NBA playoffs.

The Celtics rallied for a 121-108 victory over Miami on Friday to cut the Heat's Eastern Conference finals lead to 3-2.

But Boston struggled early, trailing by 12 points, before a 41-25 third quarter helped them turn the game.

Celtics head coach Stevens said improvement defensively was key for Boston.

"I just thought we played with great tenacity defensively, and our offense followed suit," he told a news conference.

"But they're very hard. It's easy for me to sit up and say to be at our very best and get stops on every possession.

"This is a heck of an offensive team, a heck of a well-coached team and hard to guard."

Celtics star Jayson Tatum scored 17 of his 31 points in the third quarter, with Boston going on a 20-3 run.

Miami head coach Erik Spoelstra lamented his side's inability to slow the Celtics down.

"We missed some open shots to start the quarter and then those turned into run-outs that got them going, those easy ones. This is a team that has great offensive firepower," he said.

"When they see the ball go in the basket they started to get more aggressive and putting a ton of pressure on us in the paint, much like they did in Game 3 and we weren't able to shut that off, and they just carried that through."

Game 6 is on Sunday.