Stanford shock for Radwanska
Second seed Agnieszka Radwanska suffered an early exit at the Bank of the West Classic in Stanford after she was beaten by Varvara Lepchenko in the second round.
Last year's runner-up and world No 5 Radwanska went one set down before fighting back to take the second against Lepchenko.
Radwanska, though, couldn't maintain the comeback as the world No 49 from Amercia recorded a 6-3 3-6 6-4 victory in a match lasting over two-and-a-half hours.
Top-ranked Serena Williams won in her first match since Wimbledon as she beat Karolina Pliskova 7-5 6-2.
Williams, who tried to play through illness at Wimbledon, showed signs she is on her way back to her best as she set up a quarter-final showdown with either Ana Ivanovic or Carol Zhao.
"I thought I played well," Williams said. "I'm on the right path, I'm excited and I'm looking forward to my next match."
American qualifier Sachia Vickery also caused an upset as she dispatched Monica Puig with a comeback victory to progress to the quarter-finals.
The 19-year-old world number 197 lost the first set to her Puerto Rico opponent, ranked 69, before fighting back to claim a 6-7 6-2 6-1 victory in just over two hours.
Eighth-seeded Andrea Petkovic saw off teenage qualifier Naomi Osaka 6-2 6-2 and will now meet the winner of Thursday's match between Venus Williams and Victoria Azarenka.