World number one Serena Williams beat Agnieszka Radwanska for an eighth successive time to close in on the semi-finals of the WTA Championships.
American Williams won 6-2, 6-4 in Istanbul to record her second win in Red Group with one more round-robin match to play, against Petra Kvitova.
With two wins, 32-year-old Williams is all-but assured of a place in the last four.
In the White Group, Serbia's Jelena Jankovic defeated second seed Victoria Azarenka 6-4, 6-3 in her opening match.
China's Li Na, also playing her first match, beat Sara Errani 6-3, 7-6.
The tournament brings together the top eight players in the world, with the top two players from each group of four qualifying for the semi-finals.
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