Former world number one Venus Williams has withdrawn from the WTA Tour Championship with an abdominal strain, allowing fellow-American Chanda Rubin to qualify for the elite eight-woman event in Los Angeles.

The winner of four Grand Slam titles, Williams has not played since the Wimbledon final in early July and officially pulled out of the season-ending tournament on Sunday.

She aggravated a stomach muscle injury during the semi-finals at the All England Club and has been unable to shake off the persistent problem.

Ninth on the Race to the Championship standings, Williams would have clinched one of the eight berths because sister Serena and American Lindsay Davenport had already pulled out with injuries.

Rubin joins a singles field that includes defending champion Kim Clijsters, French and U.S. Open champion Justin Henin-Hardenne, Jennifer Capriati, Amelie Mauresmo, Elena Dementieva, Anastasia Myskina and Ai Sugiyama.

The withdrawal is a blow to a tournament that suffered from poor attendance last season at the Staples Center, even with the Williams sisters taking part in the event.

Starting on Wednesday, this year's tournament features a new arrangement that has the top eight players competing in a round-robin singles format.

The Red Group consists of Clijsters, Mauresmo, Dementieva and Rubin, while Henin-Hardenne, Capriati, Myskina and Sugiyama make up the Black Group.

The top two players from each group advance to next Sunday's semi-finals with the final taking place the next evening.

The winner receives a record-tying $1 million.