LOS ANGELES -- Derrick Williams hit his first 3-pointer, then his second 2½ minutes later, completely unfazed by the yawning backdrop of Staples Center.
He kept on connecting from long range, scoring 22 points and propelling Arizona (No. 15 ESPN/USA Today, No. 16 AP) to a 78-69 victory over Oregon State in the Pac-10 tournament quarterfinals Thursday.
"It's just a confidence thing," Williams said. "Every time I make my first one, most of the time I make my second one. That's what I was just feeling throughout the game. I hit big shots when we needed it. Just trying to break the game open, and I just took good shots."
Williams, the Pac-10 player of the year in his second season, hit a career-high five 3-pointers in six attempts.
"The way he shoots the ball from 3 sometimes surprises us, and every one seemed to be timely," said Arizona's Sean Miller, the league's coach of the year.
The top-seeded Wildcats (26-6) advanced to play fourth-seeded Southern California in Friday's semifinals.
"Having the No. 1 seed, we're one step closer to the Pac-10 championship," Williams said. "It's like a home game for them. They play right up the street, so we have to match their intensity and hopefully we can."
Kevin Parrom added 13 points, helping the Wildcats end a three-year skid in the tourney quarterfinals. A year ago, they lost to UCLA and finished 16-15, ending the school's streak of 25 consecutive NCAA tournament appearances.
Now, Arizona remains on track to add its first conference tourney title since 2002 to the regular-season championship it wrapped up last weekend. Hall of Fame coach Lute Olson, who began the school's NCAA tourney streak in 1985, was in the crowd.
Lamont Jones went 3 of 6 from beyond the arc and finished with 13 points. The senior-less Wildcats had five 3s in the second half, when they pulled away after a tight opening 20 minutes.
Jared Cunningham scored 19 of his 23 points in the first half and Ahmad Starks added 17 -- both one point off their career highs -- for the Beavers (11-20), who split with Arizona in the regular season but fell to 0-5 against the Wildcats in the Pac-10 tournament.
"Their defense was really good. They closed in on me and I tried to find my teammates," Cunningham said. "Some of the shots were ill-advised and I kind of forced them."
Oregon State trailed by three at halftime. Arizona came out and hit three consecutive 3-pointers -- two by Jones and one by Williams -- to extend its lead to 10.
"If we had weathered that, maybe Joe's ejection wouldn't have been as painful as it was," Beavers coach Craig Robinson said.
Starks sank consecutive 3s that drew the Beavers within seven midway through the second half before they lost big man Joe Burton, who had nine points and five rebounds.
Burton was assessed a flagrant foul and got ejected for elbowing Parrom in the face near the baseline with 10:25 remaining.
Arizona fans chanted "Throw him out!" as the referees reviewed the play.
"I was just being physical. From what I remember, he just hit me. I fell and got back up," Parrom said.
Burton walked back to his bench claiming he got hit.
"I didn't see anything," Robinson said. "It was a physical game. I'm sure there was something leading up to that."
The Beavers trailed by nine when Burton left the court. Parrom scored six of the Wildcats' next nine points, with Williams adding his fifth 3-pointer to extend the lead to 70-56.
Oregon State began the game on a tear, seeking its first semifinal berth since 2005. They ran up a 22-15 lead, propelled by Cunningham's 15 points.
Arizona regrouped with 12 straight points, including consecutive 3-pointers by Jones and Williams to take its first lead. Williams hit two more 3s as the Wildcats closed the half ahead 40-37.