Doyle, a junior from San Francisco, dismissed Nia Rose of Oregon 6-2, 6-3 in the first round and Fabikova 2-6, 6-0, 6-3 to set up a match against No. 7 Lauren Marker of Arizona.
Also advancing to the quarterfinals were No. 1 Maegan Manasse of Cal, No. 4 Giuliana Olmos of USC and Fremont in the San Francisco Bay Area, and Krista Hardebeck of Stanford.
Hardebeck, a senior from Santa Ana in the Los Angeles area, has won 17 consecutive singles matches. It's the longest winning streak in a season by a Cardinal since Nicole Gibbs won her last 17 matches in 2012 en route to the first of her two straight NCAA singles titles. Gibbs is now ranked No. 74 in the world.
Hardebeck, though, will face No. 3 seed and defending champion Catherine Harrison of UCLA.
Doubles play begins today. Manasse and Denise Starr are seeded first, and Taylor Davidson and Doyle are second.
Stanford's Carol Zhao, last year's NCAA singles runner-up to Jamie Loeb of North Carolina, is playing doubles only in the Pac-12 Championships.
Meanwhile, the Cal and Stanford men reached today's semifinals with 4-0 victories in the team competition. The No. 3-seeded Bears blitzed No. 6 Washington, and the No. 4 Cardinal whipped No. 5 Oregon.
Cal (16-5) will play No. 2 USC (15-5), and Stanford (14-9) will take on No. 1 UCLA (20-2), featuring 10th-ranked Mackenzie McDonald of Piedmont in the Bay Area.
PAC-12 WOMEN'S CHAMPIONSHIPS
In Ojai, Calif.
(Northern California results only)
First roundMaegan Manasse (1), Cal, def. Madison Westby, USC, 6-2, 6-4.
Desirae Krawczyk (8), Arizona State, def. Alaina Miller, UCLA/Saratoga, 6-4, 6-4.
Giuliana Olmos (4), USC/Fremont, def. Alyssa Tobita, Oregon, 6-2, 6-0.
Taylor Davidson (5), Stanford, def. Nuria Ormeno Ruiz, Colorado, 6-7 (8), 7-6 (3), 6-2.
Zoe Katz, USC, def. Lynn Chi, Cal, 6-1, 6-3.
Krista Hardebeck, Stanford, def. Devin Chypyha, Arizona, 6-2, 6-1.
Kassidy Jump, Arizona State/Granite Bay, def. Sarah Richter, Utah, 6-1, 6-1.
Catherine Harrison (3), UCLA, def. Denise Starr, Cal, 7-6 (4), 7-6 (4).
Caroline Doyle, Stanford/San Francisco, def. Nia Rose, Oregon, 6-2, 6-3.
Klara Fabikova (2), Cal, def. Elianne Douglas-Miron, Washington, 6-2, 6-4.
Second roundMaegan Manasse (1), Cal, def. Annabell Andrinopoulos, Colorado, 6-1, 6-0.
Giuliana Olmos (4), USC/Fremont, def. Kyle McPhillips, UCLA, 6-4, 6-4.
Ebony Panoho, Arizona State, def. Taylor Davidson (5), Stanford, 6-4, 6-3.
Krista Hardebeck, Stanford, def. Zoe Katz, USC, 7-5, 7-5.
Catherine Harrison (3), UCLA, def. Kassidy Jump, Arizona State/Granite Bay, 7-5, 6-2.
Caroline Doyle, Stanford/San Francisco, def. Klara Fabikova (2), Cal, 2-6, 6-0, 6-3.