BOSTON -- LeBron James and the Cleveland Cavaliers only needed one quarter to elbow their way back into the lead of their Eastern Conference semifinal with the Boston Celtics.

James scored 21 of his 38 points in the first quarter to help Cleveland beat Boston 124-95 on Friday night, handing the Celtics their worst home playoff loss ever and taking a 2-1 lead in the best-of-seven Eastern Conference semifinals.

Showing no ill-effects of an elbow problem that was the talk of Cleveland during the long layoff, James finished with eight rebounds and seven assists. The 21 points in a quarter was a franchise postseason record.

"I know I'm going to hear a lot about the elbow, but I'm here to play basketball and give our team a chance to win," James said. "We knew how important it was to come out and play aggressively after giving away Game 2."

Antawn Jamison had 20 points and 12 rebounds for Cleveland, while Shaquille O'Neal added 12 points and nine rebounds.

Rajon Rondo, who had 19 assists in Boston's Game 2 victory on Monday, had 18 points and eight assists.

The Celtics missed 10 of their first 13 shots, hitting just 27 percent in the first quarter to spot Cleveland a 21-point lead. The Cavaliers shot 67 percent from the field in the first -- and 59 percent for the game -- and Boston was never able to recover.

"It was great to see LeBron set the tone from the jump," Cleveland coach Mike Brown said. "And the rest of the team followed."