Fifth seed Venus arrived at the $585,000 Advanta Championships in Philadelphia battling her sister Serena, Jennifer Capriati and Vera Zvonareva for the two remaining berths in the $3 million event in Los Angeles which starts on Monday.
Eight players will compete in California and Wimbledon champion Maria Sharapova, world number one Lindsay Davenport, U.S. Open winner Svetlana Kuznetsova, French Open victor Anastasia Myskina, Elena Dementieva and Amelie Mauresmo are already guaranteed a place.
Zvonareva, who opened her campaign with a three-set win over Austrian qualifier Sybille Bammer on Monday, must win the Advanta title to clinch a spot at the season-ending tournament.
Venus and Capriati are likely to have to reach the final to secure a place.
Six-times grand slam champion Serena, who decided not to compete in Philadelphia, should still qualify because she holds a commanding 265-point lead over Venus, who is next in line.
In other matches on Tuesday, seventh seed Nadia Petrova of Russia pounded compatriot Tatiana Panova 6-2, 6-0 while Croatia's Jelena Kostanic beat lucky loser Anne Kremer of Luxembourg 6-4, 2-6, 7-5 to set up a second-round clash with Sharapova.
