The 18-year-old Dane earlier cruised through with a 6-2 6-2 drubbing of Spanish 20th seed Anabel Medina Garrigues.
However, her older sister Venus was forced to withdraw ahead of her match against Spain's Anabel Medina Garrigues due to a back injury.
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However Spanish tenth seed Anabel Medina Garrigues became the first high-profile casualty of the tournament, losing 7-5 6-3 to 98th ranked Australian Anastasia Rodionova.
In the other half of the draw, second seed Svetlana Kuznetsova received a walkover when unseeded Spaniard Anabel Medina Garrigues pulled out with a back injury.
Reddy used to take me and Anabel out on the sled, and sometimes other kids, too, and he had a special whistle that scared off bears.
Meanwhile, Marion Bartoli of France beat Anabel Medina Garrigues of Spain 6-2 6-1 and seventh-seeded Marlene Weingartner of Germany eased past Aranxta Parra of Spain 6-2 7-5.
Kirsten Flipkens beat Anabel Medina Garrigues 7-6 (7-5) 1-6 6-2 in an opening rubber that lasted more than two hours in the crushing heat of south Spain.
Jo-Wilfried Tsonga put France ahead as he beat Fernando Verdasco 7-5 6-3, but Anabel Medina Garrigues leveled it for Spain with a 6-3 6-2 win over Mathilde Johansson.
But Williams has also hit a bump or two along the way, including a tight three-setter against Anabel Medina Garrigues in Madrid (Garrigues won the second set at love).
Safarova didn't leave Charleston empty-handed, however, securing her first WTA doubles title alongside Russia's Anastasia Pavlyuchenkova as they beat Spaniards Anabel Medina Garrigues and Yaroslava Shvedova 5-7 6-4 10-6.
But 25th seed Anabel Medina Garrigues of Spain crashed out 1-6 6-4 6-0 to South African Chanelle Scheepers, while two-time grand slam champion Svetlana Kuznetsova, seeded 26th, lost 1-6 7-6 7-6 to Czech Iveta Benesova.
Spanish 26th seed Anabel Medina Garrigues went through 6-3 6-3 against Czech Eva Birnerova, while Slovakian 20th seed Daniela Hantuchova recovering from dropping the opening set to Uzbekistan-born American Varvara Lepchenko to win 4-6 6-3 6-2.
They also won the mixed doubles to complete a third successive victory in Group A and set up a clash with Spain's Fernando Verdasco and Anabel Medina Garrigues, who won all three of their Group B matches.
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