BOSTON — It was a night of promise that turned out broken.

After a steady and confident start, the Cavaliers wilted under some intense pressure from the Boston Celtics and let Game 5 and their footing in the heated Eastern Conference semifinal series slip away in the process when a late rally to recover came up short.

Getting a big second-half performance from their stars, the Celtics returned to dominance on the their home floor in a 96-89 win and a 3-2 lead with a chance to close it out Friday night in Cleveland.

In the midst of what appeared to be a great opportunity to strike a vital blow, the Cavaliers seemed to forget about how they got back into the series. That and the Celtics started playing like the No. 1 seed that is perfect at TD Banknorth Garden in the postseason.

The Cavaliers lost their defensive intensity and became timid and tentative on the offensive end, a double whammy they couldn’t survive. Combine that with the Celtics stars and some role players stepping up and it turned into just another misfire in Boston.

For quite some time, LeBron James looked like he was going to have one of those superspecial nights and maybe guide the Cavaliers to another Game 5 road win — as he’s done in each of the past two seasons. James did have his best scoring game of the series, but he wasn’t the distributor that he can be when at his best.

James scored 23 points in the first half and made eight of his first 11 shots as his jumper finally showed some rhythm and he was able to get to the basket for high- percentage looks. At one point in the second quarter the Cavaliers owned a 14-point lead. Yet James and his teammates couldn’t sustain it.

He finished with 35 points, the most by any player in the series, but had just five assists. He was absent from making impact plays — and so were his teammates — for much of the second half as Boston built a 12-point lead that was too much to overcome.

The Cavaliers cut the margin down to four points in the final minute, but never seriously challenged as they paid for their midgame collapse.

“I felt I was going to play well, I was able to attack and get into the lane,” James said. “It was a pretty good performance by me but us losing the game means more than anything.” James was simply unable to punish the Celtics for their extra attention as he did so well in Game 4. With James as the leader, the Cavaliers had 53 combined assists during their two wins in Cleveland. On Wednesday, the offense — as it was for much of Games 1 and 2 — was shut down by the Celtics’ pressure.

The Cavaliers didn’t move the ball and did a lot of dribbling as they were able to rack up just 11 assists. They also turned the ball over 16 times, including the first three possessions of the second half, which jump-started a huge Celtics’ third-quarter surge.

“We did not move the ball after they turned up the aggression,” Cavaliers coach Mike Brown said. “We have to stay poised against their aggression and not get away from what we do.”

After the first quarter looked like an extension of what happened at Quicken Loans Arena, the Celtics started whipping passes to beat the Cavaliers’ rotations, and they had plenty of options to go to.

Just as James played his best offensive game of the series, so did opposite number Paul Pierce, who had 29 points as he was able to find driving space on the Cavaliers’ defense for the first time all series. Kevin Garnett added to his hot shooting by hitting 12-of-19 shots, most of them jumpers, for 26 points with an impressive 16 rebounds.

Then there was the real difference-maker, Celtics point guard Rajon Rondo. After mostly just shooting jumpers — his weak spot — in Cleveland, Rondo attacked Delonte West and Daniel Gibson with his quickness. Whether it was with the help of picks or simply on his own, Rondo was able to get into the paint on the Cavaliers and break down their defense.