MIAMI -- The biggest postseason comeback in Miami Heat franchise history wasn't enough.
The Heat needed more -- and got it, digging deep to take a 2-0 lead in the Eastern Conference finals.
LeBron James scored 34 points and grabbed 10 rebounds, Dwyane Wade scored 23, and the Heat rallied from 15 down to beat the Boston Celtics 115-111 in overtime on Wednesday.
Mario Chalmers scored 22 for the Heat, who won despite an unbelievable night by Rajon Rondo. The Celtics guard played all 53 minutes and scored 44 points, dished out 10 assists, and grabbed eight rebounds.
Paul Pierce scored 21 points, Kevin Garnett added 18, and Ray Allen 13 for Boston.
Allen's 3-pointer with 34.3 seconds left tied the game at 99-all. James missed two shots, first a layup and then a jumper on the final possession of regulation, and to overtime they went.
Game 3 is Friday in Boston.
The Heat had come back from 14 points down in playoff games twice before, first in Game 6 of the 2006 NBA finals -- their title clincher -- and again last season against Philadelphia.
And this one was slipping away, more than once. James also missed two free throws 21 seconds into overtime, and Miami looked to be in trouble.
The scoring dossier in overtime began like this: Rondo scored, Heat tied it, Rondo scored, Heat tied it, Rondo scored, Heat tied it.
When Rondo missed a layup with 1:33 left, Miami took advantage, with Udonis Haslem getting a dunk to put the Heat up 105-103. And after a turnover on the next Boston possession, Wade drove the lane, hit the deck, and watched as his layup bounced on the rim and dropped through.
Garnett stood over Wade and glared, to no avail. Wade hit the free throw, and Miami was up 110-105 with 59.7 seconds left.
On a night where the Heat missed 16 free throws -- including at least four by James -- they would survive.
Miami was down by 15 in the first half and by as many as 11 in the third quarter, before a pair of 3-pointers by James started a comeback.