Serena could potentially face sister Venus in Sunday's final
Top seed Serena Williams surged into the quarter-finals of the Bank of the West Classic in California with a straight-sets defeat of Melinda Czink.
The Australian Open and Wimbledon champion set up a meeting with Samantha Stosur of Australia with a hard-fought 6-3 7-6 (9-7) win over the Hungarian.
Venus Williams, the second seed, beat Russia's Alla Kudryavtseva 6-1 7-5 to set up a match against Maria Sharapova.
Should the sisters progress, they will face each other in Sunday's final.
"My error count was a little high," said Venus Williams. "I could have been a little more consistent and maybe won the second set 6-2 or 6-3.
"My key is to execute my game as best I can," she added looking ahead to her match against Sharapova.
"I feel like it will be a good match-up for me. I always enjoy playing against Maria."
In other matches, fourth seed Jelena Jankovic had to come from a set down to beat a stubborn Sabine Lisicki before winning 2-6 6-2 6-4.
Jankovic will now meet eighth-seeded Marion Bartoli of France after last year's losing finalist beat American teenager Melanie Oudin 7-5 6-4.