The top eight in the world rankings will battle to be season-ending champion
Serena Williams heads to the season-ending Sony Ericsson Championships in Doha looking to regain the number one spot from Russia's Dinara Safina.
The American slipped down to second in the latest rankings after just two weeks in top spot.
Williams will play sister Venus, Elena Dementieva and Svetlana Kuznetsova in the Maroon Group in the round-robin.
Safina has been drawn with Caroline Wozniacki, Jelena Jankovic and Victoria Azarenka in the White Group.
The tournament begins on Tuesday and the top two in each group will make the semi-finals, with the champion taking home US$1.55m.
Serena Williams must win one more match than Safina if she is to regain the world number one ranking - if both players have the same results, the Russian will stay on top.
Last year was definitely a monumental moment. I never thought it would feel so good to actually win the year-end championships
"It would be awesome," Williams said about the prospect of ending 2009 as number one. "It would be really cool. I'm in a tough part of the section of the draw, so we'll see what happens."
Safina has won only three matches in her last four tournaments but regained the top ranking this week as the points from last year's season finale were dropped.
She lost all three of her round-robin matches 12 months ago and has come in for criticism throughout 2009 for being number one despite never having won a Grand Slam title.
"I don't care any more, they can say whatever they want," said the 23-year-old.
"I have been in three Grand Slam finals and I've been in two semis. Last year I was winning every tournament and they were asking me why I'm not number one.
"This year I became number one, there is no Grand Slam. It's not that I don't want to win a Grand Slam. But you're not a robot that you can play 100 per cent all the time."
Serena Williams has been among the most vocal critics having won the Australian Open and Wimbledon titles, taking her Grand Slam collection to 11.
The American is still under investigation by the WTA following her outburst during the US Open semi-final and could yet face a Grand Slam ban.
Absent from the line-up are US open champion Kim Clijsters, who is ranked 18th as she has only played four tournaments since coming out of retirement, and Maria Sharapova, who is 15th in the world as she makes her way back after a nine-month injury lay-off.
Venus Williams is the defending champion, while Azarenka and Wozniacki are competing in the event for the first time.
"It's a really difficult tournament to win, and it gives me more experience," said 19-year-old Dane Wozniacki.
"I think it's just a great thing to be a part of this team of the eight best women tennis players in the world. I'm sure that this will help me in the further tournaments."
Venus Williams, who is also playing in the four-team doubles event with her sister, said: "Last year was definitely a monumental moment. I never thought it would feel so good to actually win the year-end championships.
"It was definitely a surprised feeling at the end. I would love to feel that again."
Opening day schedule (Tuesday from 1400 GMT):
Victoria Azarenka (BLR x6) v Jelena Jankovic (SRB x8)
Elena Dementieva (RUS x5) v Venus Williams (USA x7)
Serena Williams (USA x2) v Svetlana Kuznetsova (RUS x3)