Williams, the world's top-ranked player, won for the 45th time in 46 matches by disposing of Simona Halep 6-0, 6-4. Williams is seeking her ninth championship of the year as she prepares for the U.S. Open.
Li is the defending champion of this tournament and seeded fifth this week.
The other semifinal will send Jelena Jankovic against Victoria Azarenka. Jankovic breezed by Roberta Vinci 6-0, 6-4 and Azarenka fought off Caroline Wozniacki 6-3, 7-6 (7-5). Jankovic won this event four years ago.