Venus Williams remains on course to reach the final of the Silicon Valley Classic for the ninth time, 20 years after her first appearance in the decider of the WTA Premier event.
Williams lost to Lindsay Davenport in the final in 1998 and 1999, before gaining revenge by beating her compatriot to the title in 2000.
She overcame Kim Clijsters to win again in 2002, before losing to Davenport and Clijsters in 2004 and 2005 respectively.
Williams then lost finals to Marion Bartoli in 2009 and Johanna Konta two years ago, but she remains a force to be reckoned with.
Seeded third at the age of 38, Williams defeated Britain's Heather Watson 6-4 4-6 6-0 on Thursday to reach the quarter-finals.
Awaiting her will be Maria Sakkari, who thrashed eighth seed Timea Babos 6-0 6-1.
Fifth seed Mihaela Buzarnescu fared better, overcoming Amanda Anisimova 7-5 2-6 6-1.
Ajla Tomljanovic battled past Magdalena Frech to win 6-4 3-6 6-3, the lucky loser failing to progress despite benefitting from the withdrawal of defending champion Madison Keys due to injury.