Serena Williams of America defeated Russia's Svetlana Kuznetsova 7-6, 7-5 on the opening day of the WTA Championships in Doha after sister Venus, the defending champion suffered a shock defeat by Russia's Dementieva. Eleven-time grand slam champion Serena, is bidding to end the year top of the WTA rankings for the first time since 2002. Both Williams sisters should have been celebrating victories but Venus, was thrown off track by an inspired Dementieva who ended a five-year wait for only her third career victory against the American in 12 meetings.