Coco Vandeweghe battled into the Bank of the West Classic quarter-finals, while Monica Puig suffered a shock loss in Washington.
A one-time runner up in Stanford, Vandeweghe overcame a slow start to record a 2-6 6-4 6-2 win over Nicole Gibbs at the WTA Premier event on Thursday.
The American fourth seed had to dig deep to overcome her compatriot, completing the win in just under two hours.
Vandeweghe will face Alison Riske in the last eight after the latter defeated Ana Bogdan in similar fashion, winning 3-6 6-2 6-4.
Catherine Bellis will face Venus Williams in the quarter-finals after her 4-6 6-4 6-3 win over Sachia Vickery.
In the bottom half of the draw, second seed Dominika Cibulkova beat Urszula Radwanska 7-6 (7-3) 6-3 to set up a quarter-final with Misaki Doi, with the winner to meet either Johanna Konta or Zheng Saisai.
In Washington, third seed Puig crashed out of the Citi Open after going down in straight sets to Lauren Davis.
Puig, who had reached the semi-finals in two of her previous four tournaments, went out in the second round after a 6-4 6-2 loss.
Kristina Mladenovic breezed past Sabine Lisicki 6-3 6-3, and Risa Ozaki battled her way to a 6-1 5-7 6-4 win over Naomi Broady to reach the last eight.
Meanwhile, Camila Giorgi advanced to the quarter-finals courtesy of a walkover against Austrian Tamira Paszek.