By The Sports Xchange

BOSTON — Coming off double-digit losses the previous two games, the Boston Celtics talked about playing with more aggression in their regular-season finale against the Miami Heat with Eastern Conference playoff seeding on the line.

It took the Celtics until the second half to unleash their aggression.

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Isaiah Thomas scored 21 points to lead five Boston players in double digits, and the Celtics outscored the Heat 60-26 in the second half to overcome a 26-point deficit and prevail 98-88 at TD Garden.

Boston posted the largest comeback win in the NBA this season.

“We just had to decide that we were going to play differently and we were going to get back to being ourselves a little bit,” Celtics coach Brad Stevens said. “That didn’t mean that we were going to make every shot, turn them over or they were going to miss open shots. But it meant that we were going to compete at a different level than they were.”

With the Celtics’ win, Atlanta’s loss and the Charlotte Hornets’ win, Boston (48-34) will be the fifth seed in the East and open the playoffs against the fourth-seeded Hawks this weekend.

Joe Johnson and Hassan Whiteside each had 19 points for the Heat, who still secured the Southeast Division title and the third seed in the East because of Atlanta’s loss. The Heat (48-34) will open the playoffs against the sixth-seeded Hornets.

“You have to give a great deal of credit to Boston,” Heat coach Eric Spoelstra said. “They really stepped up their pressure, our offense got flattened out, we were playing in mud, screening, moving the ball. I looked up there in the fourth quarter and we had less than 20 points in the second half. Even our open looks, which there weren’t many, we didn’t knock those down. We didn’t have a stand defensively.”

The Celtics had lost by 14 points to Charlotte on Monday and by 11 to Atlanta on Saturday. With players from the Celtics’ 1966, 1976 and 1986 NBA championship teams, including Bill Russell, on hand to be honored at halftime, Boston never led Miami in the first half and was behind 62-38 at halftime. The Celtics did not want to let down some of the most famous names in franchise history.

“That’s embarrassing to have all these legends in this building and us playing this way,” Boston guard Avery Bradley said. “And in the second half, I saw a few of them smiling. It got everybody smiling on the bench. That’s what it’s all about. Going out there and playing Celtic basketball. We’re all a family.”

Johnson’s 3-pointer at 11:25 at the third quarter put the Heat up 65-41, but Miami didn’t score again for 7:51.

The Celtics went on a 20-0 run to cut the lead to 65-61 before Johnson’s jumper with 3:34 left in the quarter ended the streak and put Miami ahead 67-61 when the Celtics called timeout.

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Boston limited the Heat to 2-of-20 shooting from the field in the third quarter and outscored Miami 25-5 to trim the lead to 67-63 when the quarter ended.

Tempers flared after the horn when Celtics guard Marcus Smart was knocked down by Heat guard Goran Dragic on a rebound and Smart tried to get at Dragic.

Players from both sides poured onto the floor, but no punches were thrown and Smart and Dragic were assessed technical fouls.

Boston kept rolling early in the fourth quarter. Kelly Olynyk’s three-point play with 9:39 left capped a 9-2 run and gave the Celtics their first lead at 72-69.

The Celtics extended their advantage to six several times and grabbed their first double-digit lead at 89-79 on two Thomas free throws with 2:55 left.

“We just sat there and talking in this tenor and said, ‘Who do we want to be?'” Stevens said about his team’s halftime meeting. “There’s guys sitting around this building that hung banners. Like how do you want to play? How do we want to feel about ourselves? And we just looked different after that.”

The Heat got out to an early 25-9 lead with a 13-0 run capped by a Dwyane Wade basket that at 3:59 of the first quarter. Seven seconds later, Bradley ended the streak with a basket with that ended 4:08 without a point for the Celtics.

Miami was on top 35-13 at the end of the first quarter after holding the Celtics to 20.8 percent shooting.

The Heat twice took a 26-point lead in the second quarter and went to the locker room up 62-38.

“Regular season is over,” Wade said. “You use everything as a learning experience and move on. We played 82 games to get into the playoffs, and we played 82 games to get what seed we were going to get.”

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NOTES: Miami F Udonis Haslem (sprained left foot) did not get off the bench, though he warmed up and was available. … Heat F Justise Winslow, who missed one game because of a sprained ankle, returned and scored seven points against the Celtics in 28 minutes. … F Dorell Wright, who played in China this season and signed with the Heat on Tuesday, was available Wednesday but didn’t play. … Celtics co-owners Wyc Grousbeck, Steve Pagliuca and Robert Epstein presented the Red Auerbach Award to G Isaiah Thomas, who led the team in scoring at 22.2 points per game. The award is given to the “Celtics player who best exemplifies the spirit of what it means to be a true Celtic through performance on the court and off.” … For the second time in Heat history, they have four players with 1,000 points in one season: G Dwayne Wade (1,392), C Hassan Whiteside (1,021), F Chris Bosh 1,010 and G Goran Dragic (1,000). The only other time was the 2000-01 season when F Anthony Mason, F Brian Grant, G Tim Hardaway and G Eddie Jones did it. … With 10 blocked shots, the Heat ran their season total to 531, breaking a franchise record of 524 set in 1999-2000.