SEATTLE - It's not a tournament without an upset, right? Friday night at KeyArena, the Colorado Buffaloes pulled off the biggest upset in tournament history, knocking off the No. 1 seed Oregon State Beavers 68-65 in the quarterfinals of the 2015 Pac-12 Women's Basketball Tournament.

No. 9 Colorado becomes the lowest seed to defeat either a No. 1 or No. 2 in the tournament's history.

On the strength of hot starts to both halves, the ninth-seeded Buffaloes had just enough in the tank to hang on for the upset. Big plays down the stretch by seniors Lexy Kresl, Jasmine Sborov and Jen Reese provided exaclty the kind of contributions required to topple the top seed.

The Buffs advance to the semifinals, where they face the winner of California-Washington in the semifinals on Saturday night.

From the opening tip, Colorado came to play. If the Beavers were the favorite entering the tournament, somebody forgot to tell the Buffaloes. They stormed out of the gates like Ralphie at Folsom Field and rode Kresl's hot hand to an early cushion. The senior point guard drained three three-pointers in the first three minutes to stake the Buffs to an early 14-7 lead.

"It definitely helps to start strong, but we were really focused coming into the game," Kresl said about her fast start. "We just came out with a lot of intensity, and that just came from being confident and knowing that we could win this game."

All told, CU as a team started off 7-for-10 from the field. The women from Boulder rode the wave all the way to the 6:15 mark of the first half, where their lead peaked at 30-20 after Lauren Huggins hit a three from way downtown.

That seemed to be the wake up call the regualr-season champion Beavers needed. After Huggins' deep three, OSU shifted into high gear and floored it to halftime. A 13-0 run over the final six minutes put the Beavers on top at the half 33-30.

[Related video: Colorado's Jen Reese: 'We threw the first punch' in upset of top-ranked Beavers]

Despite that tough finish to the half, Colorado reemerged from the bowels of KeyArena unaffected. After being held scoreless in the first half, Reese rattled off eight straight points to put her squad back in the lead 40-39 a little more than five minutes into the final period.

"The credit goes to all my teammates," Reese said of her offensive outburst. "They gave me great passes and you've just got to find open gaps and knock down the shot when you're open."

When Jamee Swan's desperation jumper from just inside the three-point line banked in moments before the shot clock buzzer rang out, it started to feel like an upset was in the air. The Buffs led the Pac-12 regular-season champs 48-45 with just over 10 minutes to play.

When Reese swished a fadeaway jumper in the corner while getting fouled, sank the free throw and silenced the throng of Beaver fans, KeyArena was in full-on upset alert mode. Colorado had extended its lead to 56-49 with the finish line in sight.

At that point, based on the kind of season the Buffaloes have had, head coach Linda Lappe felt like her team was more than prepared to finish what they started.

"By the time you get to this point of the season, you're tested," Lappe said. "You have to rely on your seniors. Lexy, Jen and Jasmine have been in so many different scenarios in their four years that they took it in stride."

Knocking off a one-seed is never easy, though. When Ruth Hamblin converted an and-1, the Buffs found themselves behind 60-59 with a little more than four mintues to play. Hamblin's clutch play got the Beaver faithful buzzing once more and had head coach Scott Rueck believing his team could escape.

"60-59 coming out of the timeout felt like an opportunity for us to separate," Rueck said. "We didn't take advantage of that. They made big plays down the stretch and hit big shots."

[Related video: CU coach Linda Lappe after taking down No. 1 see Oregon State]

But when it's your night, it's your night. Swan restored the lead to 61-60 for the Buffs with a silky-smooth jump shot from the elbow. Then, after Gabriella Hanson could not finish at the rim for OSU, Sborov corraled the rebound and went coast-to-coast for a layup. 63-60 Buffs with less than two to play.

The final piece of the puzzle fell into place when Kresl appeared out of nowhere in the paint after a smart cut and laid in two more. With 1:06 on the clock, Colorado maintained its 65-60 lead and held on to complete the biggest upset in the history of the tournament.

How did it happen? The key, according to both coaches was OSU's inability to hit threes. The Beavers hit 38 percent of their triples this season, the best number in the Pac-12 and seventh best in the entire NCAA. On this night, they finished at 19 percent (6-for-31).

"We wanted to take away Jamie Weisner and Sydney Wiese's threes," Lappe said. "We thought that was a huge key for us, not letting them get open shots."

One cold shooting night doesn't seem to sway Rueck's belief in what his team does best.

"We did not have a great shooting night tonight and ultimately that was the difference in the game," Rueck said. "But a three is like a layup for us, that's been true all year. It's always going to be a big part of what we do."

The win snaps a 17-game losing streak against ranked teams for the Buffs. They will now appear in the tournament semifinal for the second time and improves their overall Pac-12 tourney record to 5-3. They have won at least one game in each of the four tournaments they have appeared in.

"We're playing our best basketball and we have for a few weeks now," Lappe said. "I'm just really, really happy for our entire team and our university. It's all about surviving and we survived. Now we get to move forward."