Romanian Sorana Cirstea knocked third seed Anna Chakvetadze out of the Family Circle Cup.
Williams cruises past Dulko, Cibulkova forced to retire through injury
Romanian Sorana Cirstea recorded her first ever victory over a top 10 player when she knocked Anna Chakvetadze out of the Family Circle Cup.
The unseeded 18-year-old followed up her opening round win over Vania King with a 6-2 1-6 6-2 upset of the struggling Russian.
Chakvetadze, seeded third in Charleston, is now just 4-5 in her last nine matches, with three of those defeats against players ranked 66th or lower.
She was not the only big name to be humbled in the early stages of the tournament.
Michaella Krajicek was also on the wrong end of an upset result, the 16th seed going down to 6-3 3-6 6-4 to Alize Cornet of France in the opening round.
Slovakia's Dominika Cibulkova, the 15th seed, is also out after being forced to retire with a back injury when trailing 6-4 1-0 in her first-round encounter with Russian Anastasia Rodionova.
Serena progress
However, Serena Williams had no problems in her second round clash with Argentina's Gisela Dulko, the American running out a comfortable 6-3 6-4 winner.
It is a 15th win of the year for Williams who is playing for the first time since capturing her 30th WTA Tour title at the Sony Ericsson Open in Florida.
Marion Bartoli, the sixth seed, also progressed although she had to work far harder against Australian Casey Dell'Acqua, eventually winning 7-5 4-6 6-1.
Defending champion Jelena Jankovic, who is also the top seed, gets her defence underway against American qualifier Alexandra Stevenson on Wednesday.
Maria Sharapova will also be in action when she takes on American Bethanie Mattek in her opening match. She received a bye through to round two.
