The second seeded Martens advanced on Thursday via a walkover after last year’s runner-up Monica Niculescu withdrew due to a shoulder injury.
In the third quarter-final, fifth seed Lesia Tsurenko abruptly ended the impressive run of Romanian teen Aryna Sabalenka 6-1 6-1.
Meanwhile in the evening session, American Alison Riske went down to little-known Romanian Mihaela Buzarnescu.
Buzarnescu, ranked No. 57 in the world, took the first set in a tiebreaker, before claiming the second set easily, cruising to a 7-6 (7-5) 6-1 win.
She will play Tsurenko in the other semi-final.