Garbine Muguruza booked her spot in the WTA Bank of the West Classic semi-finals on Friday, and she was joined by Coco Vandeweghe, Madison Keys and Catherine Bellis.
Top seed Muguruza was in fine form against Ana Konjuh, cruising to a 6-1 6-3 victory in Stanford.
Hoping to make her second WTA final of 2017, after defeating Venus Williams at Wimbledon, the Spanish world number four served impeccably.
With a 69 per cent first serve rate, Muguruza was dominant, winning 82 per cent of those points.
She will face Madison Keys, who defeated Ukrainian seventh seed, Lesia Tsurenko, 6-4 6-3.
Coming off wrist surgery, the quarter-final victory was the first time Keys won consecutive matches since March, where she made the Round of 16 at Indian Wells.
Meanwhile, Vandeweghe made light work of Russian fourth seed, Anastasia Pavlyuchenkova, winning 6-2 6-3.
The American sixth seed took just over an hour to secure her place in the semi-final, but Pavlyuchenkova will rue six double faults and only winning 38 per cent of second serve points.
The biggest shock of the day came in the late game, with two-time Wimbledon winner Petra Kvitova losing 6-2 6-0 to Bellis.
The 18-year-old American stepped up when it counted, winning seven of nine break point opportunities.
However, Kvitova's 26 unforced errors far outweighed her 12 winners in a disappointing performance.