British number three Heather Watson lost to seven-time Grand Slam champion Venus Williams in the last 16 of the Silicon Valley Classic in California.
American third seed Williams, 38, won 6-4 4-6 6-0 to reach the quarter-finals and is the highest remaining seed.
Defending champion Madison Keys withdrew on Thursday with a wrist injury, while top seed Garbine Muguruza pulled out earlier in the week.
World number 14 Williams will face Greece’s Maria Sakkari next.
Meanwhile, British number two Katie Boulter is out of the Citi Open in Washington after losing in three sets to qualifier Allie Kiick in the last 16.
Boulter, 22, who knocked out fifth seed Aleksandra Krunic in the first round, lost 4-6 7-5 1-6 to American Kiick.