The Celtics’ Tony Allen and Kevin Garnett celebrate as Boston closes in on last night’s season-opening victory at The Garden.

AP / Winslow Townson

BOSTON — Paul Pierce couldn’t hold back his emotions.

Neither could Kevin Garnett or Ray Allen.

After all, Pierce said, Boston’s Big Three, who have 35 years of NBA experience between them, dreamed since they were kids of earning an NBA championship ring.

Last night, in front of a sellout and celebratory crowd, their long-time wish came true as NBA commissioner David Stern gave the Boston Celtics their championship rings in an emotional Opening Night pregame ceremony that concluded with the Celtics hoisting up their NBA-record 17th championship banner and Pierce giving a heartfelt, teary-eyed speech.

It wasn’t a big surprise that the emotional Celtics began the game cold against the Cleveland Cavaliers. And Cleveland took full advantage by beginning the game with a 12-4 run as Boston missed 10 of its first 13 shots.

But once the Celtics got their emotions under control, they rallied and defeated upset-minded and much-improved Cleveland, 90-85.

It was a great way for the Celtics to open up their 2008-09 campaign, Boston coach Doc Rivers said. The Celtics hope to successfully defend their title and win an unprecedented 18th NBA championship this season.

“The key for me is to make sure that each player plays his role and does his job completely, and if he does that then I’m fine,” Rivers said. “That’s the key for me.”

If Rivers can get the most out of his players, he believes the Celtics will repeat as champions.

The Cavs believe that they are going to be one of the teams standing in the Celtics’ way of repeating as champions.

“The next step for me is winning an NBA championship,” Cleveland superstar LeBron James said. “If we can continue to get better as a team and I can continue to get better as an individual, then we’ll have a chance to compete for that. We’ll see what happens. … We have high expectations here.”

Pierce led an emotionally charged Celtics team with 27 points, 4 assists and 3 rebounds, while point guard Rajon Rondo added 14 points, 6 assists, and 5 rebounds. Garnett tallied 11 points and 6 rebounds.

Boston led by as much as eight points in the fourth quarter, but Cleveland did not go down quietly. Mo Williams buried a 3-pointer to cut Boston’s lead to 86-83 with 1:17 remaining in the game.

Leon Powe (13 points, 2 rebounds) then missed a five-foot jumper for Boston, but the Celtics got a stop on Cleveland’s ensuing possession when they forced James into a tough shot at the rim. Rondo got the rebound for Boston and the Celtics called time to set up a play for Pierce.

Pierce, however, missed a 16-foot jumper with 16.9 seconds left and Cleveland got the rebound and called time to set up a potential game-tying 3-pointer.

Instead of going for the tie, James got the ball and drove to the basket. Pierce fouled him and James missed the front-end of two pressure free throws with 10.6 seconds left. He made the second to cut Boston’s lead to 86-84, but then the Celtics wisely ran a fast break and threw the ball up court to Powe, who was all by himself under the basket. He stuffed it home and was fouled.

Powe missed the free throw and Rondo fouled James on the rebound. James, however, hit only one of two free throws and the Celtics held on for the victory after two Ray Allen freebies.

“They are a good team,” James said.

Boston allowed Cleveland to shoot 46.9 percent from the floor in the first half, but in the third quarter, the Celtics displayed the suffocating defense that made them the best team in the league last year and took control of the game.

Boston held Cleveland to just 35.3 percent shooting from the field in the momentum-swinging third quarter, when they outscored Cleveland, 24-13, and never looked back.

Pierce sparked a 12-2 third-quarter run with a 3-pointer. He capped it with two free throws that gave Boston a 55-52 lead.

Cleveland didn’t immediately go away and regained a 63-59 lead on three consecutive scores by James, but the Celtics closed out the third quarter with an 8-0 surge to go ahead, 67-63.

James (22 points, 7 rebounds, 6 assists), Zydrunas Ilgauskas (15 points, 8 rebounds) and Williams (12 points, 3 assists) were Cleveland’s top players last night.

roblee@projo.com