BOSTON — LeBron James scored 21 of his 38 points in the first quarter to help the Cleveland Cavaliers 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.

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.

They were never able to recover as the Cavaliers shot 59 percent from the field for the game.

Game 4 is Sunday and the Celtics need a victory to split their home games and assure themselves of another.

James wore a black sleeve on his right elbow, which became a concern after he shot a free throw left-handed in the closing minutes of Cleveland's first-round playoff game against Chicago. He was 8 of 10 from the field in the first quarter — most of them mid- to long-range jumpers — and after that it didn't matter.

"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.

The Celtics had high hopes after taking Game 2 in Cleveland to swipe the home-court advantage from the 61-win Cavaliers. But Cleveland earned it back in Game 3, needing just one quarter to silence the Boston crowd that got no solace when the Red Sox and Bruins also fell behind early.

"We knew how important it was to come out and play aggressive after giving away Game 2," James said.

James scored eight straight points to make it a 10-point lead midway through the first, and seven points during an 11-0 run that made it 36-15 with 19 seconds left in the quarter. Cleveland led by 24 points in the second quarter, 30 in the third, and never by less than 20 in the entire second half.

Celtics coach Doc Rivers pulled most of his starters with 6:37 left, and much of the TD Garden crowd left as well. James sat down with 5:41 remaining, giving him a chance to rest his strained and bruised right elbow.

Notes: The national anthem was sung by sandal-wearing Grateful Dead guitarist Bob Weir. ... The Celtics had a moment of silence for Bobby' Vines, a 45-year-old sales account executive who died after a lifelong battle with sickle cell anemia. ... The NBA record for points in a quarter of a playoff game is held by Sleepy Floyd, who had 29 in 1987 for Golden State. ... James reached 2,000 playoff points in his career. ... Paul Pierce, who scored 11 points on 4-of-15 shooting, didn't make his first basket until midway through the second quarter.