The Serbian, the No. 1 seed at the season-ending tournament, secured a berth in Saturday's semifinals with a 7-6 (8-6), 6-4 victory over No. 6 Svetlana Kuznetsova of Russia in the White Group.
"In the beginning for a couple of games I played very well, but then I kind of stopped," said Jankovic, who hurt her back midway through the match.
Kuznetsova sank to 0-2 and has been eliminated from semifinal contention, along with Russia's Vera Zvonareva.
In the Maroon Group, reigning Wimbledon champion Venus Williams, the seventh seed, recorded a 5-7, 6-1, 6-0 win over her younger sister, Serena, the third seed. In another Maroon Group match, fifth-seeded Elena Dementieva of Russia downed compatriot and No. 2 seed Dinara Safina 6-2, 6-4.
Sunday's champion will collect $1.34 million.