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Rajon Rondo, giving Gerald Green the slip on a drive to the net, gave the Celtics a spark after a sloppy first quarter.

NEWARK - In two distinct ways, Rajon Rondo showed how critical he is to the Celtics’ offense Saturday at Prudential Center, with three stellar quarters in a gritty victory over the New Jersey Nets.

Rondo was erratic in the first 12 minutes, committing four turnovers as the Celtics trailed early. He responded with 11 assists and zero turnovers in the final three periods of the 94-82 victory. The Celtics also turned up their defensive intensity in the last three periods, while Kevin Garnett collected another double-double (21 points, 12 rebounds) as Boston rebounded from its listless performance Friday in Toronto.

The victory also clinched a playoff spot for the Celtics for the fifth straight season. The berth was secured when the Bucks lost to the Pacers.

It was Rondo’s 21st consecutive game with 10 or more assists, a career-best streak. During that stretch Rondo has avoided the questionable passes and the shoddy decision-making that frustrates coach Doc Rivers and his teammates. Saturday night was the rare occasion that he turned a poor start into a brilliant finish.

“Just miscommunication really,’’ Rondo said, explaining the sloppy first quarter. “I wasn’t happy with my play, but I knew the team needed me and it was bigger than myself, so I tried to go out there and do what’s best for my team and run the show. Once we got on the same page, guys knew where they were supposed to be and I delivered the ball where it’s supposed to be and we connected.’’

Rondo makes it sound so simple and in the final three quarters he made it look so simple.

In a stretch of 1 minute 35 seconds of the third quarter, Rondo found Avery Bradley three times in the same spot in front of the Nets’ bench and three times Bradley knocked down the 3-pointer to help extend a once-precarious lead to 72-59.

Bradley had hit nine 3-pointers in his previous 57 games but Rondo trusted him enough to keep feeding him the ball.

“Efficiency, man,’’ Kevin Garnett said about Rondo. “When [No.] 9 is at his best man, it’s pretty hard to beat us. I’m just being honest, not being arrogant or cocky here. When [Rondo] is on his one-two, he’s making precise decisions. He’s aggressive at the basket. He’s getting rebounds. When he’s like that, nine out of 10 times we win those games. He re-gathered himself after the first quarter and never looked back.’’

As the Celtics’ offense began gaining confidence, their defense stopped Deron Williams’s dribble penetration and turned New Jersey into freelancers. The Nets made 12 of 19 shots in the first quarter and 20 of 68 (29.4 percent) the rest of the way. They were stymied by the improved interior defense. And as the Nets’ offense faltered, apathy set in and the Celtics extended their lead to as many as 17 in the final quarter.

Brandon Bass added 18 points and 11 rebounds off the bench while Bradley finished with 18 points.

The Nets were frustrated by the Celtics’ defense and the officiating, which was bizarre at times. Coach Avery Johnson and Williams picked up early technical fouls, and the Nets seemed befuddled by the officiating all evening.

The Celtics were more physical off the glass and outrebounded the Nets, 51-42. Garnett’s jumper with 31.2 seconds left in the third extended the lead to 74-59 as New Jersey began losing interest.

“[The Celtics] played harder, played better, just took us out of our rhythm,’’ said Williams, who missed 9 of 12 shots, but added 14 assists. “You definitely have to give credit to their defense. They are a good defensive team and our offense wasn’t very good, our execution wasn’t very good.’’

The Celtics made adjustments on defense to start the second quarter that were effective the rest of the game. They prevented the Nets’ big men from breaking to the basket for easy layups and New Jersey became almost exclusively a perimeter team.

Bradley scored 5 points during a 17-8 Celtics’ run to begin the second period and they worked arduously to take a 39-36 lead after an apathetic start. The lead was stretched to as many as 4 after a Bass layup with 48.6 seconds left, but the Celtics broke down defensively, allowing an open 3-pointer from Anthony Morrow.

After another Bass bucket extended the lead to 48-45, former Celtic Gerald Green canned a 31-footer at the buzzer to tie the game at halftime.

“We were just playing in random too much,’’ Rivers said of the first quarter. “Whenever we get caught into that game, you turn the ball over. I thought from the middle of the first quarter on, we played through our system and Rondo knows our system as well as anybody.’’