Top-seeded Venus Williams booked her semi-final berth at the WTA hardcourt tournament in Stanford, California, with a 6-4, 6-1 victory over 17-year-old wild card CiCi Bellis.
Ranked seventh in the world, the 36-year-old Williams on Friday 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.
Bellis, playing in her first WTA quarter-final was buoyed by an early service break.
But former world number one Williams quickly regained the break and broke again with a backhand winner down the line to secure the opening set.
It was all Williams in the second set as she moved a step closer to a third Stanford title to go with those she won in 2000 and 2002.
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.
Cibulkova's victory ensures she will return to the top 10 in the world rankings for the first time in more than 18 months.
Her 2016 campaign has included two titles and a run to the quarter-finals at Wimbledon.
"Today was really, really tough. She was playing fast from the first ball, but once I found out what I needed to do to win I was doing the right thing all the way through the match," said Cibulkova, who won the Stanford title three years ago.
"I know the court, I know the balls, I know what I have to do to go all the way here," she said.
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