BOSTON — The Celtics sent LeBron James and the Cavaliers back to Cleveland to admire all their regular-season accomplishments.

It’s Boston that is still in the chase for an NBA title.

Kevin Garnett scored 22 points and added 12 rebounds, and Rajon Rondo had 21 points and 12 assists as Boston beat Cleveland 94-85 in Game 6 on Thursday night.

The Celtics advanced to the Eastern Conference finals, which begin Sunday in Orlando. The Magic are 8-0 in the playoffs after sweeping Charlotte and Atlanta in the opening rounds.

Despite his fifth career playoff triple-double, James is headed for another early offseason after winning a second MVP award and leading the Cavs to an NBA-best 61 wins and a home-court advantage they never got to use.

This offseason is destined to define the future of the franchise — and the rest of the NBA, too.

The LeBron watch began at 10:53 p.m., when Rondo dribbled out the last 14 seconds and the Celtics began celebrating. James is eligible to opt out of his contract this summer, a move that would make the two-time MVP — and zero-time NBA champion — a free agent and set off a scramble for his services from New York to Miami to Los Angeles and, of course, back in Cleveland.

“We know their team is fueled by one guy (James), and he has a (great) supporting cast,” Garnett said. “We knew that if we could somehow, someway control that supporting cast, then we had a good chance of beating this team.”

James had 27 points with 10 assists, and his 19 rebounds tied a career high and were the most he’s had in a playoff game. He also had nine turnovers.

“I just told him, ‘Keep your head up, man. I’ve been there,’” said Garnett, who toiled as a star without a title in Minnesota for more than a decade before winning one with Boston in 2008. “‘You have a very, very, very bright future. Continue to work and make decisions based on you and your family.’ Best advice I can give him.”

Mo Williams had 20 of his 22 points in the first half for the Cavaliers.

Boston’s Paul Pierce scored 11 of his 13 in the second half after playing just nine minutes — and shooting 1 for 5 — in the first half because of foul trouble. The Celtics missed their first eight 3-point attempts before Pierce hit one that gave them a 65-58 lead with 4:06 left in the third.

It was 67-61 when Rasheed Wallace hit a 3-pointer, and then Ray Allen stole James’ pass and got the ball to Pierce for another 3 that completed a 16-4 run.

James hadn’t made an outside shot before hitting back-to-back 3-pointers to cut it to four points, 78-74, early in the fourth quarter and force the Celtics to call a timeout. But Rondo drove for a layup, then set up Pierce for another 3. Pierce found Wallace for a 3-pointer and then Tony Allen’s steal led to a Garnett dunk that sent the Cavaliers into a timeout to regroup, down 88-74 with 5:53 left in their season.

“You knew it was coming at some point with LeBron,” said Celtics Coach Doc Rivers, who reminded his players they weren’t good enough to take over the game.

“That’s what that timeout was about, to remind them that we can’t do that, what LeBron was doing.”

It was the second straight year Cleveland has finished the regular-season with the No. 1 overall seed, and the second in a row they have failed to get out of the East. Last year, they lost to Orlando in the conference finals.

James seemed like he couldn’t wait to slip off his Cavaliers jersey, pulling it off as soon as he reached the tunnel to the locker room and flipping it to an attendant after he walked into the dressing room.

“The fact that it’s over right now is definitely a surprise to me,” James said. “A friend of mine told me, ‘I guess you’ve got to go through a lot of nightmares before you realize your dream.’ That’s what’s going on for me individually right now.”