The fifth-seeded Hercog is the highest-seeded player among the final four after topping fourth-seeded Dominika Cibulkova of Slovakia, 6-3, 6-1. Martinez Sanchez, seeded sixth this week, took out third-seeded German Julia Goerges, 7-6 (8-6), 6-1.
Hercog is trying for her second title of 2011, having captured the Swedish Open in July. She also reached the final at Palermo the following week, losing to Spain's Anabel Medina Garrigues.
Next up for Hercog will be Voskoboeva, who eliminated Russia's Vera Dushevina on Friday with an easy 6-2, 6-3 triumph. Voskoboeva has never won a WTA title and will play in her second semifinal of 2011. She was beaten by Ksenia Pervak at the Baku Cup in July.
Martinez Sanchez has one title this year, taking the clay court event in Bad Gastein back in July. She'll next meet Zakopalova, who knocked out American Vania King, 6-2, 4-6, 6-1, on Friday.
Zakopalova has two career WTA titles, both coming in 2005, and nine runner-up finishes. She lost last year's Seoul title match to Alisa Kleybanova. Saturday will mark her third semifinal of 2011. She lost to Jarmila Gajdosova in Hobart to start the year and fell to Roberta Vinci in Budapest back in July.
This week's champion will earn a first prize of $37,000.