World number 22 Li spearheaded China's breakthrough year in women's tennis in 2006 but last played in June at a Wimbledon warm-up tournament, where she sustained a rib injury during a coughing fit brought about by her pollen allergy.
An ankle injury caused Zheng, the 129th-ranked singles player in the world, to miss her Wimbledon doubles title defence as well as the ongoing U.S. Open.
"It is very bad news," Chinese Tennis Association (CTA) vice president Gao Shenyang told China Daily.
"Their rehabilitation is slower than we expected, I am not sure when they will be back."
World number one Justine Henin-Hardenne, Amelie Mauresmo of France, defending champion Svetlana Kuznetsova and Swiss Martina Hingis are the leading names in the draw for the $600,000 tournament, which begins on Sept. 17.
