The 21-year-old has flourished this season, winning three tournaments, in Amelia Island, Bali and Shanghai and reaching four other semi-finals in Paris Indoors, Toronto, New Haven and Moscow.
Dementieva joins world number one Kim Clijsters, second-ranked Justine Henin-Hardenne, Jennifer Capriati and Amelie Mauresmo in the eight-player round robin singles field at STAPLES Center in Los Angeles.
World number three Serena Williams and number four Lindsay Davenport both qualified for the championships, but have withdrawn because of injuries.
With Dementieva's place now secure, there are six players -- Anastasia Myskina, Venus Williams, Ai Sugiyama, Chanda Rubin, Nadia Petrova and Vera Zvonareva -- battling for the remaining three singles spots.
All those players, except Venus, are entered in this week's Advanta Championships in Philadelphia, which is the final chance to clinch a place.
Only 413 points separate the six players.
Depending on bonus points, Myskina is likely to qualify if she avoids an early round upset.
Sugiyama and Rubin could earn spots if they at least reach the quarter-finals, while Petrova and Zvonareva have only slim mathematical chances of earning tickets to Los Angeles.
This year's WTA championships, from November 5-10, features a new format, with eight players competing in a round robin singles competition that pays the winner $1 million, equalling the largest prize in women's sport.
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