CLEVELAND — LeBron James always soars in the NBA playoffs.

Mo Williams finally rose to the occasion — literally.

Williams delivered his first dunk for Cleveland, a resounding slam that stunned Boston's Paul Pierce, shook the arena and ignited the Cavaliers, who rallied for a 101-93 victory over the Celtics last night after being outplayed for much of Game 1 of the Eastern Conference semifinals.

Williams' unexpected dunk over Pierce fueled a game-ending 43-24 spurt by the Cavs.

"I knew Mo could dunk," James said of his teammate, generously listed as 6-foot-1. "I told Mo a long time ago if he ever dunked in a game it was going to spark us like we haven't been sparked before. Not only did it fire the team, it fired himself up."

Hours after receiving his second straight MVP award, James scored 35 points and Williams added 20, 14 in the third quarter.

James, playing with a sprained and bruised right elbow, delivered yet another memorable performance as the Cavs withstood a furious punch from the Celtics, who led by 11 in the third.

James, who also had seven rebounds and seven assists, drained a 3-pointer with 22 seconds left to seal it.

"It's been better," James said of his elbow. "But that's no excuse for me. I don't make any excuses and I'll be ready for (Game 2) Monday."

Historically, winning a series opener is a good omen for the Cavs, who are 10-0 when taking Game 1.

Rajon Rondo had 27 points and 12 assists and Kevin Garnett finished with 18 points and 10 rebounds for the Celtics, who were held to 15 points in the fourth quarter.

Dismissed as being washed up, the 17-time NBA champion Celtics controlled the tempo from the start and were on the verge of swiping home-court advantage away from Cleveland.

But James, who seemed to be bothered by the elbow early on, picked it up down the stretch.

"You knew it was coming," Celtics coach Doc Rivers said. "I thought toward the end of the second quarter he started his assault."

Cleveland trailed 69-58 when Williams grabbed a loose ball and raced down the floor before slamming it over Pierce, who was probably expecting the guard to try a layup.

"I saw him jump," said Williams, whose struggles in the postseason last year hurt the Cavs. "Paul is 6-7, 6-8. I thought he'd block a layup. I was kind of high. I thought I could try. It turned out good. It was a great feeling, I'll tell you that."