Maria Sharapova withdrew from Stanford's Bank of the West Classic with an arm injury, while Garbine Muguruza impressed in her first match since winning Wimbledon last month.
Sharapova, 30, pulled out ahead of her second-round match in California against Ukraine's Lesia Tsurenko.
The Russian beat Jennifer Brady 6-1 4-6 6-0 on Monday, having previously been out with a thigh problem since 16 May.
Spain's Muguruza, 23, thrashed 17-year-old American Kayla Day 6-2 6-0.
It was the world number four's return to action following her second major triumph at Wimbledon three weeks ago.
"[Wimbledon] is just another tournament," said the Spaniard.