Oklahoma coach Lon Kruger had every reason to have confidence at the Big 12 tournament.

The Sooners had rolled into the Sprint Center with three straight wins. They'd had by all accounts a solid week of practice. And they were facing on fresh legs a team that had been pushed in the second half by TCU the previous night.

So much for all of that.

Baylor raced out to a 21-point lead early in the second half Thursday night, and then held on after a frantic rally by the No. 17 Sooners for a 78-73 victory and a spot in the semifinals.

"I thought we entered the ball game in the right frame of mind," said Kruger, the Big 12 coach of the year. "I liked our guys' fight to get back into it, and we had a little bit of a look at it. But they deserved it more than we did."

Cameron Clark scored 19 points and Buddy Hield had 15 for the second-seeded Sooners (23-9), who got within 72-68 on Hield's 3-pointer with 1:27 left, and had a chance to get even closer when Royce O'Neale turned it over for Baylor. But a 3-pointer by Jordan Woodard with about a minute left was off the mark, and O'Neal came through with a pair of free throws.

Oklahoma had a couple of open layups on the ensuing possession, but opted to toss it out to Clark for a 3-pointer. He missed it, the Sooners had to foul again and Brady Heslip — one of the best free-throw shooters in the Big 12 — knocked down two more to help the seal the win.

"We just got too comfortable, it feels like," Clark said. "We have to come out aggressive from the start, like we have nothing to lose."

Isaiah Austin scored 18 points for the No. 7 seed Bears (23-10), who became the first lower-seeded team to win in this year's tournament and will play Texas on Friday night.

O'Neal added 12 points and 10 rebounds for the Bears. Kenny Chery finished with 12 points and seven assists, and Heslip also finished with 12 points.

"That's when we're at our best," Chery said, "when we're sharing the ball."

It was the ninth win in 10 games for Baylor, which has gone from NCAA tournament bubble to knocking on the door of the Big 12 title. The Bears are in the semifinals for the fifth time after ending a four-game skid against the Sooners that included a pair of losses this season.

The Sooners were tied for the Big 12 scoring lead at better than 82 points per game, but they shot just 38 percent in the first half and were thoroughly dominated on the glass.

Baylor took advantage of the cold start to score the game's first 10 points. It maintained that comfortable advantage until the closing minutes of the first half, then used a 10-2 run that helped the Bears carry a 47-31 lead into the locker room.

The lead grew to 21 points in the opening minutes of the second half, the Bears capping a surge with an alley-oop dunk by Austin that brought their smattering of fans to their feet.

Oklahoma finally started to show a pulse, though.

Isaiah Cousins hit a 3-pointer and Clark scored three straight baskets, the last on a stylish dunk in transition. By the time Tyler Neal scored on a nifty scoop shot in the paint, the Sooners had embarked on a 13-2 run.

The Bears answered with a flurry of their own, but a basket by Cousins with about three minutes left and a 3-pointer by Hield — who had been 1 for 7 from the arc at that point — got the Sooners within 69-64 and started to give coach Lon Kruger's boys reason to believe.

Baylor was solid from the foul line down the stretch, though, and will now try for its third win in as many days in Friday night's semifinal game.

"To me, this is more wide open than it's ever been," said Baylor coach Scott Drew, who picked up his 201st win to tie Bill Henderson's school record. "That's what makes this year's tournament so exciting. It doesn't matter who we play. We're just glad to be playing."