Klay Thompson scored 25 points and Washington State upset No. 18 Washington 87-80 on Sunday to pick up a signature win in its quest for an NCAA tournament bid.

Reggie Moore added 18 points and five assists for Washington State (15-6, 5-4 Pac-10), which held high-scoring Washington to 37 percent shooting, and broke a four-game losing streak to its rival.

Isaiah Thomas scored 19 points for Washington (15-5, 7-2), which fell into a tie with Arizona for first in the Pac-10. Justin Holiday added 16 points for the Huskies, who were trying to open 8-1 in the Pac-10 for the first time since 1984.

Washington's Matthew Bryan-Amaning, who came in averaging 16 points per game, was held to six points.

Thompson, the leading scorer in the Pac-10, made 9 of 18 shots and added four assists and five steals for the Cougars. Faisal Aden scored 15, and the Cougars shot 48 percent.

Leading by two at halftime, Washington State opened the second half with a 10-1 run, including a 3-pointer and spectacular alley-oop dunk by Moore, for a 55-44 lead.

Washington missed its first five shots of the second half and did not make a field goal until Holiday's 3-pointer with 14:33 left. But the Huskies did make seven of eight free throws to close to 57-53 with 13 minutes left.

Thompson hit a lay-up and 3-pointer as WSU built a 63-55 lead.

Washington did not make its second field goal of the second half until Scott Suggs scored with 9:39 left, cutting WSU's lead to 66-60. The Huskies were 2 of 11 from the field in the second half at that point, but had made 12 of 13 free throws to stay close.

Thomas, the third-leading scorer in the Pac-10, did not make his first field goal until just 6:13 was left in the game, a 3-pointer that cut WSU's lead to 75-69.

Thompson replied with a basket and a 3-pointer for an 80-69 lead. But Thomas made another 3-pointer and a jumper to pull the Huskies within 80-74 with less than five minutes left. But they were scoreless for nearly four minutes.

Aden hit a 3-pointer to give WSU an 83-74 lead with two minutes left.

In the first half, Washington State built a 15-10 lead. Washington replied with a 12-2 run, and Terrence Ross hit a 3-pointer for a 22-17 lead. Holiday and Darnell Gant hit consecutive 3-pointers to put Washington up 32-26.

Brock Motum made a basket and then completed a 3-point play for WSU to tie the score at 37.

Thompson scored five of the Cougars' final eight points for a 45-43 halftime lead.

Washington State coach Ken Bone, a former UW assistant, won his first game in six tries against the Huskies.

Husky reserve C.J. Wilcox did not play because of a concussion.