He is so fiercely private, shifting the same passion he displays on the court to protect his personal feelings and opinions, that it is a special occasion when he unleashes, as he did following a masterful performance in the clinching Game 6 against the Hawks.

Kevin Garnett’s brilliance on the court is unquestioned, and his ability to turn insults and putdowns into motivation is why he remains not only relevant, but dominant (in stretches) at age 35.

It is a chore for the Celtics’ public relations staff to coerce Garnett to speak on the playoff podium, the place where only superstars appear. Garnett obviously still considers himself a superstar, but he is resistant to appear in such a public forum.

So when Garnett agreed to join Rajon Rondo Thursday night, he obviously had something on his mind. He basked in the 28-point, 14-rebound performance that saved the Celtics from traveling to Atlanta for Game 7, extending the Big Three era at least another series.

Polishing off Josh Smith and Al Horford wasn’t enough, he took on an even bigger Hawk in co-owner Michael Gearon Jr., who had called Garnett the “dirtiest guy in the league’’ and had said the Celtics “are old’’ and were receiving preferential treatment from the officials.

In a rare display of off-court emotion and brutal honesty, Garnett addressed those comments, defending himself and his team. While Garnett maintains he does not read the newspaper or search websites, he always manages to point out the latest media barbs about his age or rumors of his decline.

He takes such comments personally, but to acknowledge that would be admitting an outside force agitates him, and could potentially puncture that privacy bubble he has worked so hard to build.

On Thursday night, he ignored all that to take on one of his detractors.

“First off, I want to say thanks to their owner for giving me some extra gas tonight,’’ Garnett said. “My only advice to him is next time he opens his mouth, he actually knows what he’s talking about, X’s and O’s versus checkbooks and bottom lines.