LAS VEGAS - The second-seeded UCLA Bruins couldn’t have started any worse in their opening game of the Pac-12 Tournament.

They made just three of their first 18 shots to start the game. They looked up at the scoreboard at the Mandalay Bay Events Center in the first quarter and they were down by 15 points to their rivals, a young, talented USC team that came into the tournament as one of the hottest teams in the league.

It could have gone south quickly.

But shot-by-shot, possession-by-possession, rebound-by-rebound UCLA climbed back. And they ended up with a 73-66 win that vaulted them into the Pac-12 semifinals against Stanford, the winner of Friday’s late game over sixth-seeded Oregon State.

By evening’s end, all four top seeds advance to the semifinals for the first time since 2013 and for only the fifth time in the 19-year history of the tournament.

UCLA and USC split the regular-season series, the Bruins winning the Pac-12 opener and USC winning in double-overtime at home back on January 17.

USC has been playing with confidence, despite its young lineups, winning four in a row on the way to Friday night’s quarterfinal.

“Because we’re their travel partners, I watch almost every one of their games in preparation for your second opponent,” UCLA coach Cori Close said. “To see their improvement and their composure today, you know, give a lot of credit to them.”

It was a horrific start for UCLA. At one point, they were down 16-2 and then 20-5. But the steady ascent back into the game began late in the first quarter. And then it picked up steam with a 20-6 run that got the Bruins within one.

“They were the aggressor,” Close said. “They were getting everything they wanted, the kind of touches they wanted. We were letting their guards get downhill. We were letting them get off into rebounds. And it's just, you know, a credit to them. I just think they got us on our heels.”

Halfway through the second quarter, UCLA took its first lead at 30-29. With junior forward Michaela Onyenwere leading the way, they led 37-36 at halftime.

But after halftime the shots stopped falling again, UCLA would score just nine points in the third quarter and USC went on a 12-0 run to close the period and take a 52-46 lead behind a balanced scoring effort led by freshman Endiya Rogers as fellow freshman Alissa Pili sat on the bench with four fouls.

Onyenwere finished with a game-high 26 points and 15 rebounds, a monumental effort for a team that needed every bit of it. Natalie Chou added 14 points, her scoring critical in the first-half comeback and senior point guard Japreece Dean ended up with nine points, eight rebounds and nine assists.

“Natalie was a huge spark and the reason we were able to turn that momentum,” Close said.

The Trojans played just seven players in the game. Four of them scored in double figures, led by Rogers’ 18 points. But when Rogers fouled out of the game with nearly three minutes remaining, UCLA was able to take command and close out the win.

USC is ready for what comes next, an invitation to the WNIT, where this young team will get even more experience.

“We played five freshmen, a sophomore and a senior for the last month of the season, and these kids just grew up, and they played with a lot of poise, a lot of heart,” said USC coach Mark Trakh. “The future looks good.”

Trakh was asked whether he thinks the WNIT has an opportunity to be a “springboard” for his program the way it was for Arizona a season ago.

“I just used the exact same words you used, springboard,” Trakh said. “You know, I tell the team let's use it as a springboard like Arizona did. That's what we intend to do. We're going to get a few days off and get ready for it.”

Senior Aliyah Jeune said the Trojans will go into the postseason with a “winning mindset.”

“And a lot of momentum to win the whole thing.”

Stanford survives OSU comeback
The basketball adage that it’s tough to beat a team three times in one season is an adage for a reason.

Third-seeded Stanford faced Oregon State for third time on Friday night, built a 20-point lead in the fourth quarter and then survived OSU’s last stand to close the game to single-digits before finishing off a 68-57 win that sends them to the semifinals for the 19th time.

The Cardinal (26-5) will be looking to avenge last month’s home loss to UCLA in Saturday’s semifinal to punch a ticket to another title game, and potentially another shot at powerhouse Oregon, the team it felled in last year’s tournament finale.

Junior Kiana Williams led Stanford with 23 points in a defensive battle that saw the Cardinal collect six blocked shots, six steals and hold OSU to 38 percent shooting in the second half, even as they made five 3-pointers to get back into the game. Stanford focused on Oregon State’s 3-point shooters early and it helped them hold OSU to 25 first-half points.

“She’s a March basketball player,” said Stanford coach Tara VanDerveer of Williams. “She kind of starts out, gets everybody involved, and then when it's time, she asserts herself really well.”

Stanford had two wins over Oregon State this season, both by 3-point margins.

“At the end, I thought our team was trying to make it a three-point game again,” VanDerveer joked. “I was like, ‘Wait a minute, we don’t have to do that.’”

Oregon State (23-9), who had not played in the opening round of the tournament since 2013, will wait to see if it will still have the opportunity to host a pair of NCAA Tournament games. The Beavers were No. 15 in the NCAA rankings released last week, hanging precariously on the host bubble.

“I think we deserve to host,” said Oregon State coach Scott Rueck.

Destiny Slocum and Aleah Goodman finished with 12 points each. Slocum was 4 of 14 from the floor.

The Beavers’ senior guard Mikayla Pivec finished with 10 points on 3 of 13 shooting from the field and collected six rebounds to set the school rebounding record, passing Ruth Hamblin.

Pivec was emotional after the game as Rueck honored the accomplishment of his 5-foot-8 guard, who is one of the most tenacious rebounders in the country.

“I’m just sad we lost,” Pivec said. “I wanted to be playing more games in this tournament.”

Michelle Smith is a contributing writer for pac-12.com. She has covered pro and college sports for espnW, San Francisco Chronicle, The Athletic and AOL Fanhouse. She was has won several awards, including the WBCA's Mel Greenberg Media Award, presented annually to a member of the media who has best displayed commitment to advancing the role of the media in women's basketball. For previous Michelle Smith features on pac-12.com, visit the archives page.