The 28-year-old Niculescu, currently the world number 55, defeated Jana Cepelova from Slovakia 6-4, 6-2 in 81 minutes.
Cepelova, currently the world number 102, had reached the quarterfinal by scoring a surprise victory over the defending champion Irina-Camelia Begu.
Tig, ranked the world 135, beat Sara Sorribes Tormo of Spain 7-6(2), 2-6, 6-3 in 154-minute showdown.
Niculescu will next play third seed Zhang Shuai who on Friday beat Italy's Camila Giorgi, and Tig will match against Lara Arruabarrena of Spain.