Williams, who won the inaugural China Open in 2004, got by Elena Vesnina 6-4, 6-2 despite three double faults and dropping her serve twice.
However, Williams did have five breaks and took more than half (36-of-70) of the points off Vesnina's serve.
Radwanska edged Stefanie Voegele 4-6, 6-4, 6-4 by fighting off 13 of Voegele's 17 break opportunities. Radwanska dropped her serve at least once in each set but had two breaks in both the second and third sets to advance.
Li, the local favorite, beat Daniela Hantuchova 6-0, 6-4. Li had five breaks and won more points (35-34) than Hantuchova when Hantuchova served. She dropped only 16 points in the first-set shutout.
No. 13-seeded Sabine Lisicki took out Chanelle Scheepers 6-3, 6-2 in Sunday's final match.
Other results Sunday saw Maria Kirilenko defeat Zheng Jie 7-5, 6-2; Madison Keys come out ahead of Dominika Cibulkova 6-4, 6-3; Laura Robson outlast Klara Zakopalova 7-6 (7-3), 4-6, 6-1; Misaki Doi rout Silvia Soler-Espinosa 6-1, 6-1 in 47 minutes; and Lauren Davis beat Mona Barthel 6-0, 5-7, 6-4.