Li triumphed 7-5 6-1 over No,3 seed Zheng, who held the highest rank of all remaining players after Sloane Stephens and Nadia Podoroska's first-round exits.
World No.52 Zheng struggled to win on her first serve, with Li converted five of seven break points.
American Li, who is ranked No.72 in the world, will next face Swiss qualifier Viktorija Golubic, who beat Russia's Anna Kalinskaya 6-2 6-4.
On the other side of the draw, Canadian Leylah Fernandez defeated Slovakia's Viktoria Kuzmova 7-5 6-3 and will face No.7 seed Sara Sorribes Tormo in the other semi-final.
Sorribes Tormo, who is now the highest ranked remaining player, defeated Anna Karolina Schmiedlova 6-3 7-5.