DOHA (AFP) - Serena and Venus Williams, who have qualified together for only the second time for the WTA Championships, find themselves with their most dreaded obstacle as they seek to reach the final of the women's tour climax. The highest profile sisters in sport are keen to show that, not only are they the best players of this decade, but they are in the process of joining the pantheon of all-time greats. But they have landed in the same group - which means they must play each other once this week (Nov 4-9), and will have to do it twice if they are to make it an all-Williams, all American final next Sunday. "Oh, we're always in the same half of the draw, so we're used to it now," shrugged Serena, injecting a little humour as she tried to convince everyone it was no longer a big issue. However losing a big final to a sibling can be a different matter, and Wimbledon appeared to be an agonising loss for the younger sister to take while Venus had to be careful to keep her initial celebrations muted. Both Williams have won Grand Slams titles this year - only the third time it has happened - and both are aware that at the ages of 28 and 27 there may not be much time left in which to enhance their legacies of achievement. This was the issue which two years ago made Chris Evert, one of the sports all-time great legends, write an open letter to Serena in an American tennis magazine, regretting that the younger sister was being distracted by other interests and asking whether she thought it was tarnishing her legacy. The Williams camp thought those comments harsh, and one person close to the sisters said that "there is a lot of stuff that Chris doesn't know is going on in Serena's life". Now however Serena seems to be able to give more commitment to prolonging her fitness while Venus showed she still has big ambitions beyond Wimbledon by winning the Zurich Open last month, ensuring she qualified for the WTA Championships for the first time since 2002. Both would like to atone for last year's event at Madrid, where Venus pulled out without hitting a ball because of long-term health issues, possibly anaemia, and Serena was unable to complete her first match before quitting with a leg injury. This year has thus became crucial for each to show that, despite the advance of time, they remain an important force, something which caused Serena four months ago to claim they had "decades left". Both are encouraged by the fact that this is the first time the women's year end championships has equal prize money equal to that of the men's - 4.45 million dollars having been made available by Sony Ericsson, the sponsor. But the Williams duo will also have to survive more matches against world class players than at any other tournament. Their main rivals could be the two Serbians, Jelena Jankovic, the world number one, and Ana Ivanovic, the French Open champion, and perhaps Olympic champion Elena Dementieva and world number Dinara Safina. Each player will play three round robin matches, after which there will be a semi-final and a final White group:- Jelena Jankovic (SRB), Ana Ivanovic (SRB), Svetlana Kuznetdova (RUS) and Veraq Zvonareva (RUS) Maroon group:- Dinara Safina (RUS), Serena Williams (USA), Elena Dementieva (RUS), and Venus Williams (USA).