Radwanska begins well
Sharapova, Serena win
Sixth seed Kvitova was toppled 3-6, 2-6 by Poland's Agnieszka Radwanska in a contest that saw the Czech commit a whopping 41 unforced errors to only five from her opponent in the round robin competition featuring the world's top eight women.
Wimbledon and US Open champion Williams eased to a 6-4, 6-1 victory against Germany's Angelique Kerber, while French Open champion Sharapova beta Italy's Sara Errani 6-3, 6-2.
Sick Kvitova pulls out
Later, Kvitova pulled out from the WTA Championships due to a viral illness and will be replaced by Samantha Stosur, tournament officials announced on Wednesday.
Meanwhile, Juan Carlos Ferrero's singles career drew to an emotional end when he lost 5-7, 3-6 to fellow Spaniard and close friend Nicolas Almagro in the Valencia Open first round on Tuesday.
Sixth seed Almagro, 27, Ferrero's junior by five years, had too much power and speed for his mentor in an entertaining match in which the pair exchanged smiles and jokes. The pair embraced at the net and the 32-year-old was given a rousing ovation by the crowd as he fought back tears before flinging his racket, shirt and various other personal items into the stands. .
World number five David Ferrer, who has already qualified for next month's season-ending ATP Tour finals in London, battled past Belgian qualifier Olivier Rochus 7-5, 7-5. Second seed Jo-Wilfried Tsonga's chances of qualifying for the finals were dealt a blow when he pulled out of his first-round match against Xavier Malisse with the unseeded Belgian leading 3-1 in the first set.
Yuki upsets Tursunov
Yuki Bhambri shocked third seed Dmitry Tursunov to progress to the second round of the ATP Challenger event but his doubles partner Divij Sharan went down fighting against sixth seed Yuichi Sugita in Seoul. Yuki recorded a 6-4, 7-6 (5) win over the Russian. Divij pushed his Japanese opponent hard before losing 2-6, 7-5, 4-6 in his opening round.