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Serena Williams powered past Victoria Azarenka to claim her third straight win in the Red Group of the season-ending WTA Tour Championships.
Williams fought back from 3-0 down in the second set to wrap up a 6-4 6-4 win in Istanbul and her 11th victory in 12 career meetings against the Russian.
Williams said: "I've been struggling with my serve in the first two rounds and I knew I could do much better. I knew I needed to play better to win against Victoria, who has had a stellar year."
Azarenka will now meet China's Li Na for a place in the semi-finals on Friday, after Li overcame fifth-seeded German Angelique Kerber 6-4 6-3 in a match lasting one hour and 25 minutes.
In the White Group, Italy's Sara Errani summoned up a grandstand finish to see off Australia's fading Samantha Stosur 6-3 2-6 6-0.