DOHA // The Williams sisters will extend their battle for family bragging rights into a 23rd episode tonight when they slug it out for the end-of-season Sony Ericsson WTA Tour championship in Doha. Serena, who defeated her elder sibling in a three-set marathon in the group stages of this event which brings together the world's top eight women players, will be strongly fancied to come out on top for the fourth time in succession and open a 13-10 lead in their personal rivalry record. It will be the 11th final in as many years to be contested by the two Californians. If Serena wins it she will boost her considerable earnings by $US1.55m (Dh5.7m) which includes a bonus for going through the entire week undefeated. Venus's winner's cheque will be $100,000 lower because she lost two of her three group matches on the way to the semi-finals. After Venus, the holder of the Tour title, had fought back strongly to secure her place in another final at the expense of the tenacious Serbian Jelena Jankovic, Serena completed the double act by overcoming the talented but tired Dane Caroline Wozniacki. Wozniacki, who has made tremendous strides up the world rankings this year to a proud position of fourth, had nothing left in the tank after losing a tight first set to Serena and became the fourth player to "retire" in this jinxed event. After Dinara Safina, who arrived here as world No 1 but will leave as second in the rankings behind Serena, quit after playing only two games of her opening group match, Vera Zvonareva, her replacement, also could not fulfil her commitments. Then came the failure to finish by Victoria Azarenka on Friday which confirmed Wozniacki's participation in the last four and Wozniacki herself completed the casualty list which may well grow to five during the final. Wozniacki has taken the lead role in a tournament full of walking wounded. The Danish teenager arrived here under investigation for conceding a match in Luxembourg due to injury when she was a single game from victory and she has kept the WTA trainer busy ever since. The left thigh which casued her to default a week earlier has been heavily strapped in all of her four matches in Doha and it has required treatment in all of them, not least because of the gruelling nature of her first two group fixtures. Twice she called for more medical assistance -- she needed to leave the court to have her first bout of trouble dealt with - and she will be more grateful than most of the exhausted competitors here that the winter break has come a little earlier this year and will be considerably longer than in previous years. The same can be said for Safina who began this campaign much more impressively than she finished it and needs as much time as possible to begin her recuperation from a back injury. Overall he decision by the WTA to shorten its season is one to be welcomed for the general good of the game.