Top seed and Wimbledon champion Garbine Muguruza made a winning start as she won through to the quarter-finals at the Bank of the West Classic but Maria Sharapova succumbed to injury.
Playing for the first time since capturing the Wimbledon crown at the All England Club, Muguruza eased past Kayla Day 6-2 6-0 on Wednesday.
Spaniard Muguruza was a class above as she wrapped up proceedings in just 58 minutes at the WTA Tour event in Stanford midweek.
"I think I learned a lot from that match [against Kayla in Indian Wells where she won in three sets]," Muguruza said afterwards.
"I knew that it was going to be difficult, so I really trained hard and prepared. Kayla has just won the US Open in Juniors and she's just very promising, so I knew I had to be concentrated today."
Awaiting Muguruza in the next round is fifth seed Ana Konjuh, who defeated Natalia Vikhlyantseva 7-5 6-4.
An arm injury forced former world number one Sharapova to withdraw from her match against Lesia Tsurenko.
Sharapova was sidelined for eight weeks due to a leg injury but pain in her left foreman ended her tournament prematurely.
Meanwhile, Madison Keys outlasted fellow American Caroline Dolehide 3-6 6-2 6-3.