SEATTLE – Before the game even tipped, it was already setting up to be one of the most electrifying matchups in the 16-year-history of the Pac-12 women’s basketball tournament.

By the time it ended, it was a thrilling, record-setting night that absolutely lived up to its advance billing in every way.

Kelsey Plum’s jumper near the lane was tipped away by Ruthy Hebard just before the horn sounded and Oregon pulled off the biggest upset of the 2017 Pac-12 Tournament so far, defeating third-seeded Washington 70-69 in front of the largest crowd ever to witness a women’s game in the Pac-12 Tournament.

A gathering of 9,686 packed Key Arena Friday night saw it all – offensive fireworks, dramatic momentum swings, star players stepping up in big moments and a nail-biting finish.

Ducks freshman Sabrina Ionescu, the Pac-12 Freshman of the Year, cooly stepped to the free-throw line with 6.4 seconds to go and drained a pair of free throws.

“I was not going to miss those free throws,” Ionescu said.

The Huskies got the ball back, and to no one’s surprise, it was in the hands of Plum, the NCAA’s all-time leading scorer, for the game-winning attempt.

With a foul to give, Oregon fouled Plum to burn more than two seconds off the clock and Plum’s second attempt to win the game off the inbounds was short when Hebard managed to get a hand on it, sending the Ducks into a raucous celebration after perhaps their biggest win in more than a decade.

“I missed shots late. I'm known for making shots, and I didn't make them late, and that's why we lost,” Plum said. “You can give credit to them, but as a player, as an All-American, I have to do better, I have to play better, so end of story.”

Plum, who finished with 34 points, became the third player in NCAA history to score 1,000 points in a single-season during the game. But she wanted this win more.

Oregon, the No. 6 seed, earns its first berth in the Pac-12 semifinals since 2005. They will take on second-seeded Stanford, who handily defeated Washington State earlier in the evening.

The Ducks cemented their NCAA status with their most impressive victory of the season, a game head coach Kelly Graves called a “signature win.”

“I’ve been coaching for a lot of years and I’ve never been more proud of a team that I am of the way we played tonight,” Graves said. “It was a packed house and they weren’t cheering for us.”

Washington coach Mike Neighbors said “That had to be a fun one to watch.

But a very tough one to lose, though the Huskies likely still have earned a No. 3 seed in the NCAA Tournament with their 27-5 record.

“Thank you, Pac-12, Seattle, everybody that did everything to pack that house tonight,” Neighbors said. “It was awesome.”

Neighbors said the Ducks played like a team that needed a win.

“I thought they were already in the NCAA Tournament, but they played like a team that thought they needed to have another one,” Neighbors said.

Oregon Junior guard Lexi Bando led the Ducks with 23 points including 5 3-pointers and a key stretch in which she helped to erase a 60-51 Huskies lead with 9:00 to go in the game. Bando hit 11 points during that critical run to pull the Ducks with in 64-62 with 5:13 to go.

“We just played with a lot of heart and grit,” Bando said. “It was so much fun out there.”

Ionescu ended the game with 18 points, six assists and six rebounds. She looked completely unfazed by the biggest stage she’d ever played on her in career, shrugging off the pressure she felt at the free-throw line.

“I’ve been there before,” Ionescu said.

Plum’s 34 led Washington, who also got 12 points and a tournament-record 27 rebounds from fellow senior Chantel Osahor.  Freshman Aarion McDonald added 10 points. Those three players accounted for all but 13 of the Huskies points in the game.

Key Arena was rocking as the teams were introduced, the primarily purple-clad fans streaming into the gym to see one of the most intriguing matchups of the bracket.

[Related highlights: Oregon women's basketball celebrates in Seattle following upset win over Washington]

On a day when the rest of the top seeds firmly held their grasp on their spots, Washington and Oregon promised to be a barnburner – a Pacific Northwest pairing pitting the veteran stars of Washington – Plum and Osahor, the nation’s leading scorer and rebounder, respectively – against two of its brightest young talents in Oregon’s freshmen duo of Ionescu (the Pac-12 Freshman of the Year) and post Hebard, the conference leader in field-goal percentage this season.

It was the opening game in the final Pac-12 tournament for Plum, who has never led the Huskies past the semifinals in this field. And it was the first for the superstar guard since she set the NCAA scoring record with a 57-point (also a Pac-12 single-game record) performance against Utah six days earlier.

It ended with a bitter taste for Plum, who was 15-of-33 from the floor. Plum was 9-for-21 in the second half.

“We will be ready for March,” Plum said. “I’m not going to miss like that again.”

Top-seeded Oregon State decisively won its battle against Cal, No. 4 seed UCLA held off Arizona State and No. 2 seed Stanford breeze past Washington State as the chalk ruled the day until the finale.

The tournament semifinals are set with top-seeded Oregon State facing UCLA at 6 p.m., followed by the Ducks and the Cardinal at 8 p.m.

Notes of the day: Friday night’s attendance of 9,686 shattered the previous single-session tournament record of 6,543 from last year’s semifinals – when, not coincidentally, the Washington Huskies were also playing.

With 34 points Friday night, Plum moved within 49 points of the NCAA single-season scoring record, currently owned by Jackie Stiles (1,062). She needs 41 points to tie Odyssey Sims’ (Baylor) 1,054 points for second.

Michelle Smith is a contributing writer for pac-12.com. She has covered pro and college sports for espnW, the San Francisco Chronicle and AOL Fanhouse.