Rondo

BOSTON — He’s not one of the Celtics’ “Big Three.”

In fact, at 6-foot-1 and 171 pounds, he’s not even big by NBA standards. But Boston second-year guard Rajon Rondo plays big in big games.

Last night, in Game One of the best-of-seven second-round series against Cleveland, with a lead in the series and home-court advantage at stake, Rondo was at his best.

Despite sitting for nearly the entire fourth quarter, Rondo scored 15 points, which tied a playoff career high for him, and added six assists and five rebounds to help Boston to a 76-72 victory.

The Celtics needed every one of Rondo’s points and assists because Boston All-Stars Ray Allen (no points) and Paul Pierce (four) struggled on offense, combining for 2-for-18 shooting and 10 turnovers.

“Rondo brought great offensive play, especially in the first half,” Boston center Kendrick Perkins said. “[He] stepped up.”

The 22-year-old floor general who finished fourth in the Most Improved Player of the Year voting this year, played aggressively, driving to the basket, drawing contact and getting to the free-throw line when there was no other option. He was 5-for-6 from the charity stripe.

Rondo also played great defense. He helped limit former Celtic Delonte West to four points, on 2-for-10 shooting.

Rondo established himself from the start, scoring eight first-quarter points to help give Boston a 25-15 lead after the first period.

He followed that up with seven points and two assists in the second quarter, with his first bucket of the period ending a 6-0 Cleveland run that had slashed Boston’s lead to 32-30. He then assisted on a Pierce layup that gave Boston a 36-30 advantage.

Rondo scored Boston’s last five points of the first half and had a hand in its last nine points of the half, which gave the Celts a 41-37 lead at the break.

“Rajon, in the first half especially, basically carried us,” Boston coach Doc Rivers said. “…He was terrific.”

After the most prolific first-half playoff scoring performance of his career, Rondo failed to score a point in more than 10 minutes in the third quarter, so Rivers inserted Sam Cassell to give him a breather.

Because of Cassell’s experience and his hot-shooting hand in the fourth quarter, in which he scored 12 of his 13 points, Rivers left him in the game for most of the fourth period.

Rondo returned to action with just under three minutes left in the game with the Celtics leading, 68-65, but it was just to give Cassell a brief rest.

“Rajon has done some great things at the end of games,” Rivers said. “Bottom line, it was an ugly game. I knew we were going to go to the post, and if you know you’re going to go to the post you have to have shooting on the perimeter, and that was my decision there.”

Rondo has shown great maturity in just two seasons in the NBA, Rivers said earlier this season. Boston All-Star Kevin Garnett recently called Rondo one of the team’s leaders.

Rondo has improved in nearly every category from his rookie season last year, upping his regular-season scoring average to 10.6 points from 6.9, his assists from 3.8 to 5.1 and his rebounds from 3.7 to 4.2. That’s because he spent plenty of time in the gym over the summer, Rondo said.

And hee continues to improve in the postseason.

Rondo averaged 11.6 points, 7.3 assists, 4.1 rebounds and 1.71 steals in the Celtics’ opening-round series against Atlanta.

roblee@projo.com