The 28-year-old Niculescu, currently the world number 55, trounced third seed Shuai Zhang of China 6-0, 6-4 in 74 minutes.
Playing the first event since reaching the third round of the US Open this month, the 55th-ranked Niculescu remained almost perfect throughout the battle, displaying powerful and accurate shots against the 27-year-old Zhang, ranked 40.
The other semi-final saw Arruabarrena, ranked 90th, end the run of Romania's Patricia Tig, ranked 135th, with a 6-1, 6-2 win.
The 22-year-old Arruabarrena advanced to the semifinals after brushing aside second seed Johanna Larsson of Sweden 6-4, 6-0.
The final takes place on Sunday.