Top-seeded Victoria Azarenka, playing for the first time since an injury withdrawal from Wimbledon, celebrated her birthday with a WTA win at Carlsbad on Wednesday.
Australian Open champion Azarenka, ranked third in the world, cruised into the quarter-finals with a 6-2, 6-3 second-round victory over former French Open champion Francesca Schiavone of Italy.
"Just going on the court and competing was so much joy for me," said Azarenka, who turned 24 on Wednesday.
She hadn't played since limping out of Wimbledon after hurting her right knee in her first-round win over Portugal's Maria Joao Koehler.
"I missed playing and I was just happy to be able to do something I've been training for," Azarenka said. "I enjoyed the time I had to go through rehab and learn so many new things. I was excited to see how I would handle it in the match. It was a good feeling to have that platform, and I'll just try to make it better and better now."
Former Wimbledon champion Petra Kvitova, the third seed, also eased into the quarters, beating Britain's Laura Robson 6-1, 6-2
Kvitova hadn't been counting on an easy passage.
"I didn't know exactly what to expect from myself because the first match after preparing for the US hardcourts is always difficult, plus I lost to her at the Australian Open," Kvitova said. "But this is tennis, and every day is different, and that's how I approached the match. I played well and my serve was great today. We practiced very hard on my serve back home after Wimbledon."
Fourth-seeded Roberta Vinci of Italy advanced with a 6-4, 6-2 win over American Bethanie Mattek-Sands.
Vinci will next face either US qualifier Coco Vandeweghe or former world number one Ana Ivanovic.
Ivanovic, the seventh seed from Serbia, rallied to beat defending champion Dominika Cibulkova 4-6, 6-3, 6-2 in the last match of the first round.
Cibulkova, who won the title up the California coast at Stanford on Sunday, was looking perfectly able to maintain her momentum as she took the opening set. But with the second set deadlocked at 3-3, Ivanovic came alive, winning eight games to take the second frame and build a 5-0 lead in the third.
"As soon as the draw came out I thought this was one of the toughest first-round matches, but I was really happy with my performance today, especially in the last two sets," Ivanovic said. "Since Wimbledon I've been working on consistency and hitting a lot of balls, and I did work out in the gym a lot, trying to get stronger and more stable. This gives me good motivation to keep doing that."
Fifth-seeded Australian Samantha Stosur also advanced with ease, defeating Kazakh qualifier Sesil Karatantcheva 6-4, 6-1.
Stosur's match under the floodlights on Wednesday night was preceded by Swiss great Martina Hingis's return to WTA action for the first time since her second retirement in 2007.
Hingis teamed with Slovakian Daniela Hantuchova for a 6-1, 6-1 doubles victory over Julia Goerges of Germany and Croatian Darija Jurak.