Williams has not played since beating top-ranked Victoria Azarenka in the final of the U.S. Open in September and her sparse schedule is one of the reasons she won't have a chance of overtaking the Belarusian despite dominating the major events in the second half of the year.
Azarenka needs only two wins in the elite, eight-player WTA Championships in Istanbul to end 2012 as No. 1.
Williams and Azarenka have been drawn into the same round-robin group. Play starts Tuesday.
"I don't really care who I play, when I play, as long as I can play," Williams said. "I'm really happy to be here. I'm on the Red side with Victoria, it'll be good."
Angelique Kerber of Germany and Li Na of China complete the Red Group.
French Open champion and second-ranked Maria Sharapova of Russia headlines the White Group, with former Wimbledon champion and last year's winner in Istanbul, Petra Kvitova of the Czech Republic, Agnieszka Radwanska of Poland and Sara Errani of Italy completing the bracket.
Since losing to Williams in the U.S. Open final, the last of the four Grand Slams of the year, Azarenka has won 13 straight matches and has held the No. 1 ranking for 34 weeks.
The Belarusian won the Australian Open in January and five more titles this year.
"Every match here (in Istanbul) is tough. I obviously have great champions in the group. It's tough no matter what. It's challenging, definitely, but I'm looking forward to it," said Azarenka, who lost last year's Istanbul final to Kvitova.
Williams has played a curtailed schedule because of injuries and illness but she holds a 10-1 career record against Azarenka.
The American is ranked No. 3 in the world, but she holds a 9-1 record this year against the other seven finalists in Istanbul.
Williams has only twice finished the year as No. 1 — in 2002 and 2009. She has played in the season finale six times and won twice, in 2001 and 2009, her final appearance at the event.
Her record this year is 53-4, with her only losses coming to Ekaterina Makarova in the fourth round of the Australian Open, to Caroline Wozniacki in the quarterfinals in Miami, to Virginie Razzano in the first round of the French Open and to Kerber in the quarterfinals in Cincinnati.
Sharapova briefly held the No. 1 ranking after winning the French Open.
"When you're coming to the Championships you pretty much know what you're going to get in terms of the level of the group," the Russian said. "With the field we have this year, I think every match no matter what group you're in is going to be pretty tough from the get go."
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