Victoria Azarenka was beaten by Donna Vekic at the Silicon Valley Classic, while two seeds made second-round exits on Thursday.
Azarenka went down to fifth seed Vekic 6-4 6-3 at the WTA Premier event played on hard-courts in San Jose, California.
Both players created eight break-point opportunities, but Vekic made the most of hers, converting five compared to Azarenka's three.
The Croatian moved into the quarter-finals, where qualifier Kristie Ahn awaits.
Ahn caused an upset with a 6-3 6-3 victory over Belgian third seed Elise Mertens.
Another seed, Danielle Collins, was also resoundingly beaten, suffering a 6-2 6-0 loss to Zheng Saisai.
Zheng will meet Amanda Anisimova after the 17-year-old fourth seed cruised past Madison Brengle 6-2 6-2.