CHAMPAIGN, Ill. -- While his teammates held Gonzaga (No. 18 ESPN/USA Today, No. 19 AP) at arm's length Saturday, Meyers Leonard spent much of the first half on the bench.

The 7-foot-1 center was Illinois' best match for Gonzaga's 7-foot Robert Sacre. But with two fouls, Leonard sat for the half's final 8:39.

Leonard didn't miss a beat when he returned for the final 20 minutes, leading a rotating cast of Illini big men that shut down Sacre and scoring 17 of his game-high 21 points in an 82-75 upset over Gonzaga (5-1). Illinois (8-0) remains unbeaten.

"I knew I had to come out in the second half and keep my head on straight to help the team win," Leonard, a sophomore, said. "I came out a little more confident as the game went on, and I felt like I continued to do good things. It felt good."

Sacre had 14 points with 4:22 left in the first half but didn't score again until he hit a jumper with 4:13 left in the game and finished. He fouled out less than a minute later and, with Illinois up 71-62, the game was all but over.

The Illini used a combination of Leonard, 6-foot-8 junior Tyler Griffey and a pair of freshmen -- 6-foot-11 Nnanna Egwu and 6-foot-8 Ibby Djimde -- to frustrate Sacre. Between them they had 12 fouls and, Sacre said, played a physical second half that turned the game and squelched him.

Sacre was impressed by Leonard.

"Runs the court like a deer. He's a great player," Sacre said. "I actually respect him a little more, I feel."

Sacre finished with 16 points.

D.J. Richardson had 19 for Illinois, while Brandon Paul added 13. Point guard Sam Maniscalco finished with 10 points and six assists. Afterward, the Illini were wondering what it takes to land in the Top 25.

"There are how many undefeated teams in the country now?" Weber asked rhetorically. "Somebody's got to notice."

Elias Harris led Gonzaga with 19 points.

Early, Sacre had his way, using his 14 first-half points to keep the Bulldogs close through the first half.

But after Sacre went cold -- frustrated by the rotating cast of Illini around him -- the Illini went on a 7-0 run early in the second half that gave them breathing room and broke what had been a 38-38 tie.

Griffey capped it with a short jumper with 15:57 left in the game for a 45-38 lead.

"Every time we made a run Illinois did a great job of answering," Gonzaga coach Mark Few said.

Ten minutes after Griffey's jumper, it was Richardson's turn.

First he took the ball on the break after a turnover, and drove straight toward the basket and Sacre. The ball didn't drop, but Sacre fouled Richardson -- sending Sacre to the bench with his fourth and Richardson to the line. He hit both free throws for a 64-58 lead with 6:47 to play.

Just over a minute later, Richardson buried a 3-pointer that gave the Illini a 67-60 lead.

Sacre finally found his touch again, hitting a short jumper and punching the air in relief as Gonzaga closed to seven at 69-62 with just over four minutes to play, but that relief was short lived.

On the next possession Leonard quickly hit a jump shot that erased Sacre's two points and kept the lead at 71-62.

Leonard, with 3:38 left in the game, then was fouled by Sacre.

"He just kind of threw his body into me, and kind of gave me a forearm a little bit," Leonard, in understated fashion, said.

There was nothing understated about the moment, though. The packed Assembly Hall erupted as Sacre walked to the bench.

Illinois rode the momentum of that loud home crowd and carried a lead through much of the first half. And even a stretch that looked ripe for an Illini meltdown ended with Illinois in front by eight.

Leonard, Egwu and Griffey all drew their second fouls in a short stretch and, with just under nine minutes to play, all headed to the bench. Leonard and Griffey stayed there the rest of the half.

The Bulldogs tied the game at 26 on a jumper by with 7:05 to play, and looked ready to run past an Illini lineup that included three freshmen and no one over 6-8.

But juniors Richardson and Paul carried Illinois.

First Richardson hit a three that fired up the crowd to give Illinois a 29-26 lead. Then Paul turned David Stockton's turnover into a basket, followed quickly by another 3 by Richardson, and Illinois was up 34-26 with 6:03 left in the half.

"We were able to survive it," said Weber, who played a dozen players by halftime Saturday, including five freshmen. "You could see a little nerves but I just kept saying fight and battle."

The Illini led 38-36 at the half, the first game this season the Bulldogs have trailed at halftime.

Few said the game and the crowd offered Gonzaga a real test.

"It was a great road environment for our guys, it was the first one of the year," he said. "I thought we played well enough at times."

Gary Bell Jr. left the game late with what Gonzaga coach Mark Few said appeared to be a twisted knee. He didn't return.