Fourth seed Capriati, battling Vera Zvonareva and Venus and Serena Williams for the two remaining berths in the $3 million WTA Tour Championships in Los Angeles which begin on Monday, became the first player to reach the Advanta quarter-finals.
In other second-round action, Wimbledon champion Maria Sharapova, already guaranteed a place in next week's elite eight-playerfield, also went through.
The Russian teenager progressed after her opponent, Croatian Jelena Kostanic, was forced to retire with a strained adductor muscle when trailing 2-6, 6-4, 3-0.
The other five players assured a berth in California are world number one Lindsay Davenport, U.S. Open winner Svetlana Kuznetsova, French Open champion Anastasia Myskina, Elena Dementieva and Amelie Mauresmo.
Mauresmo, the top seed in Philadelphia, begins her campaign on Thursday with a second-round clash against Iveta Benesova of the Czech Republic.