MIAMI — LeBron James got his ring, then got cramps.

The Miami Heat found a way to beat their biggest rival anyway.

Dwyane Wade scored 29 points, James finished 26 points and 10 rebounds while missing much of the second half, and the reigning NBA champion Heat beat the Boston Celtics 120-107 on Tuesday night in the season opener for both teams.

Ray Allen, in his first game with Miami since leaving Boston over the summer, added 19 points for the Heat, as did Chris Bosh, who had a late flurry that helped Miami prevail.

Rashard Lewis scored 10 for Miami, which saw a 19-point lead trimmed to four in the final minutes before finding a way to close it out — even while James, last season’s MVP of both the regular season and NBA Finals, was in the locker room for the second time because of the cramps.

Paul Pierce scored 23 points, Rajon Rondo finished with 20 points and 13 assists, and Leandro Barbosa scored 16 for Boston. The Celtics, who lost to the Heat in last season’s Eastern Conference finals, were not on the court for the ring ceremony.

They almost wrecked the festive mood with a late comeback.

An 11-2 run late in the fourth quarter got Boston within 111-107 on Courtney Lee’s layup with 2:09 left. That was the last Celtics’ hurrah — Bosh scored the game’s next seven points, sealing it for Miami.

The Heat got their championship rings from owner Micky Arison and watched the banner commemorating last season’s title get hoisted in a pregame ceremony replete with an indoor fireworks show.

More fireworks came at the finish — Rondo was called for a flagrant foul against Wade with 16.9 seconds left, as if either team needed a reminder that these sides simply don’t like each other very much. Three straight postseason meetings made them rivals, and Allen’s departure only added more fuel to the mix.

Jeff Green, who had season-ending surgery for an aortic aneurysm last January, played in a game that counts for the first time since May 11, 2011 — also in Miami, when the Heat ousted the Celtics from that season’s playoffs. Green finished with three points in 23 minutes.

As Allen entered the game for the first time, he trotted toward the Boston bench, exchanging a handshake, embrace and a few words with Celtics coach Doc Rivers, who hasn’t hidden his displeasure about his former shooting guard’s decision to sign with Miami. Allen then briskly shook hands with a few assistant coaches.

But when Allen tried to engage Kevin Garnett, the mutual pleasantries ended. Allen tapped Garnett on the shoulder as he sat on the Boston bench; Garnett didn’t even flinch, staring straight ahead, refusing to acknowledge the gesture in any way.

Allen didn’t seem flustered. His first shot in a Miami uniform was — what else? — a corner 3-pointer, which swished.

Miami controlled much of the first half, taking a 62-54 lead after James went by Pierce and Garnett for a two-handed dunk and yell for the crowd.

The first Heat angst of the season came in the third quarter, when James walked slowly to the bench after a timeout with 4:40 remaining, favoring his right leg and then headed out the tunnel toward the locker room.

When he left, the Miami lead was 79-70.

When he returned to open the fourth quarter, the lead was 93-76. The Heat scored the final eight points of the third, with Bosh, Allen, Mario Chalmers and Lewis all scoring in the final 2:17 to give the Heat their biggest lead of the game.

James — who was dealing with a cramping issue, like he experienced at times in last season’s NBA Finals — opened the fourth quarter with a jumper to extend the run to 10-0, and Miami seemed well on its way. But he returned to the locker room a short time later, the cramps having flared up again.

NOTES: Wade passed the 15,000-point mark for his career early in the second quarter, becoming the 123rd player in NBA history to reach that mark, according to STATS LLC. ... Rondo had at least 10 assists for the 25th straight regular-season game. ... The Celtics allowed 62 points in the first half; they gave up 62 or more only seven times in 379 regular-season and playoff games over the past four seasons. ... Miami players wore sneakers with gold somewhere in the color scheme; James’ were primarily gold — in honor of the ring — and trimmed in white and red.

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