The Orlando Magic's Dwight Howard turned in a powerful all-round game to lift the Magic to an 86-79 victory over the Boston.
Howard scored 18 points, grabbed 15 rebounds and blocked five shots as the Magic charged to a seven-point first-quarter lead and extended it to 18 by the half to go 51-33 in front.
The Celtics nearly made the Magic's extended 22-point lead disappear, drawing to within three points before Howard sank a pair of free throws with 13.9 seconds left to make sure of victory.
The win tightens up the race for Eastern Conference seedings.
Orlando improved to 46-16 with the victory and have just one more loss than defending NBA champion Celtics (49-15), who wasted a 32-point contribution by Ray Allen.
The defeat gave Boston two more losses than East Conference leading Cleveland Cavaliers (49-13).