Amelie Mauresmo faces a tough task to hold on to the number one ranking as she defends her WTA Championships title in Madrid this week.
Mauresmo won the title in Los Angeles last year
Mauresmo is under threat from Maria Sharapova and Justine Henin-Hardenne at the eight-woman season finale.
To stay at number one, Mauresmo must retain her title with Henin-Hardenne fourth in their round-robin group, and Sharapova losing in the semi-finals.
Mauresmo plays Nadia Petrova in the opening match at 1700 GMT on Tuesday.
Sharapova then takes on Elena Dementieva before Henin-Hardenne faces Hingis.
Mauresmo will play for the first time in a month following a shoulder injury.
"I'm feeling better," she said ahead of Tuesday's opening round of matches.
"I picked up my racket again last Monday and started serving and everything was OK.
"I haven't played since Zurich so obviously I would like to have had more matches but I'll take it as it comes.
"I might be fresher than the other girls and that might give me an edge at some point."
We are also working towards a lighter schedule for 2009
Kim Clijsters is on the way back from a wrist injury, while French Open champion Henin-Hardenne has not played since the US Open.
This has prompted Sharapova to call for wide-ranging changes to the calendar.
"I feel strongly that the tennis season needs to be made shorter, with more breaks for players to rest in order to be in peak condition when we do play," said the Russian.
The number of top-10 women pulling out of events through injury has tripled in 2006 compared to 2005 and the WTA has responded by suggesting a cut in the number of events the women should play from 13 to 11 in 2007.
WTA chairman Larry Scott said: "We are also working towards a lighter schedule for 2009, a year earlier than expected.
"The season will end at the end of October rather than mid-November as is the case today.
I'm very excited but I'm married so I can look but not touch
Henin-Hardenne on male models as ball boys
"The new calendar will also have breaks in the year to give players a respite after the big tournaments."
Tournament organisers have decided to use male models this week as an answer to criticism over the use of female models as ball girls at the men's Madrid Masters.
"I was talking to Maria (Sharapova) in the taxi and she said they looked nice," said Clijsters.
"I'm excited about it, although it is going to be strange. As long as they do a good job, that is all that matters."
Henin-Hardenne said: "I'm very excited but I'm married so I can look but not touch."
Yellow Group:(1) Amélie Mauresmo (Fra), (3) Justine Henin-Hardenne (Bel), (5) Nadia Petrova (Rus), (7) Martina Hingis (Swi)
Red Group:(2) Maria Sharapova (Rus), (6) Kim Clijsters (Bel), (4) Svetlana Kuznetsova (Rus), (8) Elena Dementieva (Rus)