The Pacers had lost a back-to-back against Houston and San Antonio earlier in the week, and they couldn't afford to lose more ground to Orlando, New Jersey and New York in the race for the seventh and eighth in the East.
"It was a good game for us, but now we have a tougher task -- and that's trying to stay in the playoffs," O'Neal said. "It just takes one day to lose to be right back out of the playoffs."
The Pacers have won their last two home games by a combined 48 points.
Shaquille O'Neal led Miami with 13 points and was the only Heat player to reach double figures. He played just 25 minutes with foul trouble.
Miami shot 32.5 percent, its lowest percentage of the season, and it was the fewest points the Pacers have allowed this season.
"We didn't have anything from the first possession, and they took advantage of it," Miami coach Pat Riley said. "They were playing a very big game, and we were just playing a game."
Indiana made 11 of its first 13 shots. A jumper by Jermaine O'Neal gave the Pacers a 29-7 lead, their last basket in a 15-0 run.
"We did not bring anything at all to the game," Riley said. "Their collective sense of urgency and desperation and the fact that they were just so good in the first quarter -- the game is over with."
Miami cut Indiana's lead to 57-42 on a bucket by Shaquille O'Neal with 8:15 left in the third quarter, but Jermaine O'Neal's three-point play with 5:07 left in the quarter bumped Indiana's lead to 64-44. The Pacers led 72-49 at the end of the period.
"We knew they were going to make a run because Pat Riley is a motivator," Jermaine O'Neal said. "They made a run in the first two minutes, but we withstood the run and got back to doing what got us the lead -- and that's sharing the ball and playing tough defense."
Dunleavy shot 6-for-10 from the field, including 3-for-4 on 3-pointers. It was the fourth time in March that Dunleavy has scored at least 20 points.
"Dunleavy's shooting has improved steadily since he's been here," Carlisle said. "He's worked on it, and he's getting more confidence with how we play."
Riley saw a different team than the one he scouted.
"I can't believe they lost 12 out of 13 games the way they played tonight," he said. "Maybe they've gone through it and probably shook it off. They looked as good as anybody we've played in a long time."
Miami has still won 10 of its last 12 games.
"They just made shots," Miami guard Eddie Jones said. "For us, nobody made shots ... and nobody defended tonight. It's a bad loss, but it's just one game. We just didn't have it tonight."
Miami is 11-5 with G Jason Williams back in the lineup after he sat out with an abdominal tear. ... Miami G Jones was averaging 14 points per game in March, but he was held to two points on 1-for-5 shooting. ... Miami reserve C Alonzo Mourning sat out with tendinitis in his right knee. He was injured Sunday in a loss to Orlando. He sat out the Heat's victory over Atlanta on Wednesday night and his status is day-to-day, a Heat spokesman said. ... Neither team had an offensive rebound in the first quarter. ... Indiana G Darrell Armstrong missed a shot in the fourth quarter and fell to the floor in front of Shaquille O'Neal, his former Orlando teammate. O'Neal playfully held Armstrong's jersey to prevent him from running downcourt to play defense.