Li, the No 1 seed in the inaugural year of the tournament, defeated eighth-seeded Bojana Jovanovski 6-3, 6-3 behind five breaks in nine receiving games. Li won 57 percent of the points on serve and 51 percent when Jovanovski served.
Peng, seeded sixth, claimed a 2-6, 7-5, 6-2 decision over Annika Beck. Peng controlled the third set, winning two-thirds of the points overall and dropping just three points in four games serving.
The other quarterfinals Thursday saw fifth-seeded Klara Zakopalova oust No. 2-seeded Marion Bartoli 6-3, 6-2 with five breaks and Monica Niculescu stop Zhou Yi-Miao 6-4, 6-2 with six breaks in the nine games Zhou served.
Li is 4-1 lifetime versus Peng while Friday's semifinal will mark the first meeting between Zakopalova and Niculescu.
Celebrity Couples of 2014 [PHOTOS]