San Francisco - Top-seeded Venus Williams booked her semi-final berth at the WTA hardcourt tournament in Stanford, California, on Friday with a 6-4, 6-1 victory over 17-year-old wild card CiCi Bellis.
Williams, 36 and ranked seventh in the world, needed only an hour to get past the 203rd-ranked Bellis in an all-American clash.
Williams, who captured the most recent of her 49 singles titles in February in Taiwan, next faces another American, Alison Riske, who was leading 6-3, 0-1 when fourth-seeded Coco Vandeweghe retired from their quarter-final with a right ankle injury.
Vandeweghe left the court in a wheelchair, pressing an ice pack to her ankle.
The other semi-final will pit second-seeded Dominika Cibulkova of Slovakia against third-seeded Johanna Konta of Britain.
Cibulkova won 11 straight games after trailing 2-5 in the opening set en route to a 7-5, 6-0 victory over Japanese fifth seed Misaki Doi.
Konta defeated China's Zheng Saisai 6-2, 4-6, 6-3.