Derrick Williams tallied 22 points and seven rebounds and top seed Arizona pulled away from ninth-seeded Oregon State in the second half to win 78-69 on Thursday in the quarterfinals of the Pac-10 tournament in Los Angeles.

Lamont Jones and Kevin Parrom scored 13 points apiece for No. 15 Arizona (26-6), which will face No. 4 seed Southern Cal in the semifinals on Friday. Solomon Hill scored nine points and grabbed 12 rebounds.

Jared Cunningham scored a game-high 23 points on 9-of-14 shooting for Oregon State (11-20), which won its first Pac-10 tournament game since 2006 on Wednesday against Stanford. Ahmad Starks had 17 points Thursday, including four 3-pointers. Devon Collier and Joe Burton added nine points apiece.

Arizona led 40-37 at the break after both teams shot over 50 percent in the first half.

Oregon State trailed by seven after Starks’ 3-pointer with 12:45 left, but the Beavers went the next five minutes without a field goal. Starks hit another 3-pointer with seven minutes left, but by then Arizona had a commanding 68-54 lead.

Williams, the Pac-10 Player of the Year, shot 8 of 12 from the field, including 5 of 6 from beyond the arc. Arizona outrebounded the Beavers 35-27 and shot 11 of 21 (52.4 percent) from 3-point range.