The first two times that Baylor played Oklahoma this season, the Bears struggled through the opening minutes and failed to get on track until it was a bit too late.

This time, they delivered the opening blow.

The Bears roared to a 21-point lead early in the second half Thursday night, and then held off a frantic rally by the Sooners for a 78-73 victory in the Big 12 tournament quarterfinals.

Isaiah Austin finished with 18 points for the seventh-seeded Bears (23-10), who became the first lower-seeded team to win in this year's tournament. Baylor will face the winner of Texas-West Virginia in the semifinals on Friday night.

"They beat us the first two games, and coach always says it's hard to beat a team three times," said Cory Jefferson, who added 14 points and 11 boards for Baylor. "I'm sure they knew that. They were prepared and we were going to make sure we were prepared."

Cameron Clark scored 19 points and Buddy Hield had 15 for the second-seeded Sooners (23-9), who spent most of the second half frantically rallying from a 52-31 deficit.

They 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.

"Baylor from the start was more aggressive, and dictated, and I thought we were reacting most of the first half," Oklahoma coach Lon Kruger said. "They played with more urgency."