Coach Doc Rivers made most of the right moves as the Celtics won their first-round playoff series against the Hawks in six games.

But, as it turned out, the most important pregame talk came not from Rivers but from a Hawks’ owner, whose bulletin-board comments provided extra motivation for Kevin Garnett to produce a 28-point, 14-rebound, 5-block performance in Boston’s 83-80 victory Thursday night.

And the Celtics’ key defensive sequence in the final seconds owed more to good fortune than what Rivers and his assistants had devised during a timeout.

Though Garnett had his most productive game since last season and Paul Pierce (18 points, seven assists) made a strong showing despite a knee sprain, this game went to the wire as the Celtics advanced to the Eastern Conference semifinals against Philadelphia, beginning Saturday at TD Garden.

Garnett took offense at being called “the dirtiest player in the league,’’ by Hawks owner Michael Gearon Jr., who had been speaking at a fund-raiser in Atlanta Wednesday.

And Garnett took his game to the basket, posting low and abandoning his pass-first mentality.

“I told [Garnett] at halftime, I counted four times where he had the ball in the post and he moved the ball,’’ Rivers said. “And I said, ‘Kevin, when the ball hits your hands I want you to hold it. And wait. And then go to work. No more being a ball mover.’ And, I thought in the second half he took that advice and he was sensational.’’

Garnett, whose highest regular-season output was 25 points against Milwaukee Feb. 29, scored half of the Celtics’ 16 points in the final quarter.

The Celtics squandered a 9-point lead in the final quarter, falling behind by 3 points with 2:24 to play.

Pierce cut the deficit to 1 with a drive with 2:04 to go. Then, Garnett’s turnaround in the lane - following a Ryan Hollins rebound - gave the Celtics an 80-79 lead with 30 seconds remaining.