German Goerges, the highest-ranked player left in the event entering the quarter-finals, was beaten 7-6 (8/6) 6-1.
Martinez Sanchez, seeded six, also beat Goerges in their only previous meeting, in New Haven in 2008.
The match featured nine breaks of serve with the opening set breaker proving crucial. After it went the way of the Spaniard, Goerges' challenge wilted and Martinez Sanchez wrapped up the win in one hour and 50 minutes.
Fifth seed Polona Hercog defeated fourth seed Dominika Cibulkova for the loss of just four games and will now play Galina Voskoboeva - 6-2 6-3 winner against Vera Dushevina.
The semi-final line-up was completed when Klara Zakopalova beat Vania King 6-2 4-6 6-1. She will play Martinez Sanchez in the last four.