Chicago ? Frank Williams had a horrible first half.

He knew it. His teammates knew it. Illinois coach Bill Self knew it.

But that’s the beauty of Williams. So maddeningly indifferent at times, he can just as easily turn it on with a dazzling display of skills that leaves opponents on the floor, gasping for breath.

He did it in the second half Sunday, scoring all 20 of his points to carry fourth-seeded Illinois to a 72-60 victory over Creighton in the Midwest Regional.

“I know the first half I didn’t play that well, and coach kind of jumped me in the locker room,” Williams said. “But my teammates kept telling me, ‘Pick it up, you know what to do out there. You’re the best player on the floor.”‘

That he is. Williams went 7-of-10 from the floor in the second half, and also had five assists and five rebounds. He also helped clamp down on Creighton’s leading scorer Kyle Korver, who was held to just six shots.

“When he wants to play, he’s as good a guard as there is in the country,” Creighton coach Dana Altman said. “That second half, Frank decided he was going to take the game over, and he did.”

Brian Cook added 16 points and Cory Bradford had 12  all on three-pointers  for the Illini, who took full advantage of their “home game.”

Illinois is 16-6 at the United Center since it opened in 1994, and it’s easy to see why. The arena was awash in orange and blue, and the fans were so loud it seemed more like Assembly Hall than Chicago.

The road gets tougher from here for Illinois, though. The Illini (26-8) will play top-seeded Kansas on Friday in the Midwest Regional semifinals in Madison, Wis.

“We’re definitely going in there with the predator mentality,” said Robert Archibald, who finished with 11 points. “We’re going in there with something to prove.”

Creighton (23-9) goes home still looking for its first second-round victory in the NCAA tournament. The Bluejays outrebounded Illinois 34-27, including a whopping 17-6 edge on the offensive glass.

But they shot just 37 percent, and didn’t have the same magic or spark they had Friday, when they stunned Florida on Terrell Taylor’s 3-pointer with 0.2 seconds left in double overtime.

“They had a little more energy than we did,” Creighton coach Dana Altman said, but added it had nothing to do with playing two OTs Friday. “We just weren’t tough enough to finish the game.”

Unlike his idol Michael Jordan, Taylor couldn’t follow up with another amazing effort. Taylor scored 10 points and was just 4-of-14 from the floor, 2-of-9 from 3-point range. Korver had 14 points.

“We were trying to beat them by ourselves, and we’re not good enough to do that,” Korver said. “We needed to use each other, and we didn’t do it.”

Williams can be as enigmatic as he is skilled, frustrating fans as he seems to tune in for some games and not others. He did it Sunday, taking just two shots in the first half and missing both.

“I just told him he’s a lot better than what he’s playing,” Illinois coach Bill Self said of his halftime “pep talk.”

“I’m more demanding on him than I am on any other player in our program,” Self added. “There are times he needs to be challenged, and halftime today was one of those times.”

Williams responded in a big way. When Korver opened the second half with a 3 to pull Creighton within 31-29, Williams took over. He scored on a reverse and then hit the first of his four 3-pointers.

After Archibald scored on a layup, Williams hit another 3 to give Illinois a 43-34 lead with 16:37 to play.

“(Korver’s 3) may have been good for our team in the long run, because I think that bothered Frank,” Self said. “He was a lot more aggressive after that.”

Illinois hit another lull and Creighton rallied, getting to 46-44 on Michael Lindeman’s free throws with 12:33 to play. But there was Williams again, hitting back-to-back 3s and then feeding Cook with a feather-soft pass under the basket.

Cook scored on the easy layup, starting a 12-0 run. Williams capped the spurt with an effortless layup, and Illinois led 64-49 with 4:15 to play.

Creighton never threatened again.

“Once again, guys realized it could be their last game. Then Frank turned the lights out,” Illinois’ Lucas Johnson said. “He decided there was no way they were going to beat us.”

Williams was solid the entire game Friday, when Illinois routed San Diego State. But he was nonexistent in the first half Sunday, taking only two shots and missing both.

The Illini were able to live with “Bad Frank” for the half, though, with Cook scoring 14 on 6-of-7 shooting in the first 20 minutes.

Besides, Williams came through when the Illini really needed him.

“You can’t afford to have that type of performance,” Williams said of his first half. “You have to be on your toes and you have to be ready for the challenge. From here on, we know it’s not going to be easy.”