Anderson Varejao of the Cavaliers, left, beats the Celtics’ Rajon Rondo to a rebound during the first half of last night’s game.

The Providence Journal / Glenn Osmundson

BOSTON — There have been several times during the playoffs that Boston coach Doc Rivers has opted to go with 15-year NBA veteran Sam Cassell late in the game instead of starting second-year point guard Rajon Rondo.

Last night, in the momentum-swinging fifth game of the best-of-seven Eastern Conference semifinal playoff series with Cleveland, Rondo made it hard for Rivers to take him out in the second half.

So he didn’t.

As a result, Rondo had a career night. He scored a playoff career-high 20 points, dished out a playoff career-high 13 assists and grabbed 2 rebounds in a playoff career-high 42 minutes, helping Boston to a 96-89 triumph over Cleveland.

“He played like a beast,” Boston forward Kendrick Perkins said. “He did his thing. He made some huge shots.”

“He’s our point guard, and that’s our leader out there,” Boston forward James Posey said.

“He’s got a quiet confidence, and I think he grew up right in front of our eyes,” Boston forward P.J. Brown said.

Even Cleveland superstar LeBron James gave Rondo high praise.

“He was the X factor,” James said. “He was the player of the game.”

Cleveland has sagged its defense on Rondo throughout the series, daring him to take shots. He made them pay in the last two games, during which he has scored 15 and 20 points, respectively, while hitting 55 percent of his shots.

“It’s not the first time that he has seen [their defense],” Posey said. “He adjusted. He attacked and made plays for everybody else. He’s so quick, sometimes with them sagging, he can still get to the basket.”

Boston forward Kevin Garnett showed the sellout crowd of 18,624 and the millions watching at home how proud he was of Rondo with just under eight minutes to play after Garnett had slammed home a dunk off a Rondo assist that put Boston up, 80-70. Boston then got a stop on Cleveland’s next possession and Garnett grabbed the rebound.

He handed the ball to Rondo and then gave him a friendly head butt to tell him how proud he was of him.

Up to that point, Rondo had scored 16 points and dished out 12 assists, with Garnett being the beneficiary of quite a few of them.

“He played great tonight defensively, and also on the offensive end making plays and controlling the tempo,” Posey said.

Rondo had the hot hand from the start. He scored Boston’s first two points on a 15-foot pull-up jumper that cut Cleveland’s lead to 4-2. After stealing the ball, Rondo scored on a fast-break layup with 1:20 remaining in the first quarter before showing his range with back-to-back 3-pointers late in the second quarter that cut a nine-point Cleveland lead to 43-37 with 1:56 left in the first half.

“I thought the two biggest shots in the game were the ones he made in the first half, those two threes,” Brown said.

“We got a couple of threes out of Rondo, so that really kind of started our run going into halftime, and it just carried over into the third quarter,” Boston All-Star Paul Pierce said.

“Those were shots that when they go in, you love them,” Rivers said. “They probably gave him confidence. He had a great feel for the game tonight. I thought he made good decisions. Getting into the paint was really big for us.”

Rondo played the entire second half, in which he scored 10 points and had 6 assists. Boston All-Star Ray Allen said that when Rondo is playing well, it takes the pressure off everyone else.

roblee@projo.com