Eighth seed Ann Li toppled the top remaining seed Zheng Saisai in the quarter-finals of the Monterrey Open on Friday, throwing the field wide open.

Li triumphed 7-5 6-1 over third seed Zheng, who held the highest rank of all remaining players after Sloane Stephens and Nadia Podoroska's first-round exits.

World number 52 Zheng struggled to win on her first serve, with Li converted five of seven break points.

American Li, who is ranked 72nd 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 seventh 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.