Seventh-seeded Jankovic took a medical timeout early in the second set but worked through pain in her right arm to set up a final match against either Maria Sharapova or Agnieszka Radawanska in the $2 million tournament.
Li broke Jankovic to go up 2-1 when the Serbian player took a timeout to have her arm treated. Jankovic broke back in the fourth game and never looked back.
"I started to feel some pain in my arm," the former top-ranked Jankovic said. "The ball was heavy in this humidity. I received some treatment and tried not think about it and just fight for every point."
Jankovic has won two titles this year at Cincinnati and Marbella.
"If I can win my third title of the year, that would be pretty good," Jankovic said. "It's been a difficult year so it would be nice to finish strong."
Jankovic is the highest seeded player left in the tournament.
Top-ranked Dinara Safina of Russia, Venus Williams and French Open champion Svetlana Kuznetsova all lost their opening matches after first-round byes.