World number one Serena Williams beat Agnieszka Radwanska for the eighth time to inch towards a semi-final place at the WTA Championships in Istanbul.
The American won 6-2 6-4 to stay atop her group standings.
Her next match will be against former champion Petra Kvitova.
In the other group, Jelena Jankovic beat second seed Victoria Azarenka 7-6, 6-4 to lead the group ahead of Li Na who had defeated Sara Errani.
World number two Azarenka must win her final match of the group to advance.
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