Rajon Rondo had seven points, 10 rebounds and 12 assists for Boston, and Glen "Big Baby" Davis scored 22 points with eight rebounds.
Boston's slim chances of repeating as the No. 1 seed in the East were erased earlier when Cleveland beat Philadelphia and clinched home-court through the conference finals. But the Cavaliers' victory helped Miami, which remained one game ahead of the 76ers for the No. 5 spot in the East.
Dwyane Wade scored 21 of his 31 points in the second half, adding nine assists, to help the Heat tie it 95-all with just under 2 minutes left. But Boston scored the next eight points to pull away.
Michael Beasley scored 23 with 13 rebounds and Jermaine O'Neal had 16 points and nine boards for the Heat.
It was the sixth straight victory for Boston, which maintained its lead over Orlando for the No. 2 seed in the East despite playing without Kevin Garnett for the sixth straight game
- and 19th of 23. The Magic played the New York Knicks later Friday.
Boston led 79-72 to start the fourth quarter before Miami went on a 16-9 run and tied it on Wade's back-to-back 3-pointers. The Heat briefly took the lead on a three-point play by James Jones, but Pierce hit a 3-pointer and the best Miami could do after that was tie it.
After O'Neal's basket with 2:46 left, Pierce hit a pair of free throws, then Wade missed a jumper before Davis made a layup on a pass underneath from Kendrick Perkins to give the Celtics a cushion. O'Neal and Wade both missed jumpers in the final minute before Pierce hit two more free throws and Ray Allen made four. Jones' 3-pointer in the last 10 seconds made it seem closer than it was.