Maria Sharapova pulled out of the Stanford Bank of the West Classic with an arm injury, while Garbine Muguruza impressed in her first match since winning Wimbledon last month.
Sharapova, 30, withdraw before her second-round match against Ukraine's Lesia Tsurenko. The Russian will also miss next week's Rogers Cup in Toronto.
She beat Jennifer Brady on Monday, having been out since May with a thigh problem.
Muguruza thrashed Kayla Day 6-2 6-0.
It was the world number four's return to action in California following her second major triumph at Wimbledon three weeks ago.
"[Wimbledon] is just another tournament," said the Spaniard.
"I know it might sound weird - because of course winning it gives me confidence - but it doesn't mean that I'm going to come here and because I just won Wimbledon I think it's going to be easier to win matches. It's not at all.
"I'm just working on trying to forget what happened and just starting Stanford. Stanford is my goal, and that's it."
Sharapova is ranked 171st after returning to tennis in April following a 15-month doping ban.
The five-time Grand Slam champion played eight matches on clay before injury forced her out of the grass-court season.
She was given a wildcard to play in the US for the first time since March 2015 at Stanford, and will require another wildcard to make the main draw at the US Open, which starts on 28 August.
"Toward the end of the Monday night's match, I felt pain in my left forearm," said Sharapova.
"After yesterday's scan, the doctor has recommended I don't risk further injury."