Top seed Halep has not lost a set in the inaugural clay-court tournament in her homeland after the world number three cruised to a 6-3 6-1 victory over Lara Arruabarrena in the quarter-finals.
The in-form Halep rose to her career-high world ranking after reaching the semi-finals at Wimbledon and she stayed on course to put the disappointment of losing to Eugenie Bouchard at the All England Club behind her with an impressive performance.
Halep had beaten Arruabarrena each of the two meetings between the pair and the Spaniard was unable to buck that trend, as the 22-year-old went a break up by winning the third game and then broke again to take the first set after only giving her opponent one opportunity to break.
A ruthless Halep looked in a hurry to seal her place in the last four as she won the first four games of the second set and despite Arruabarrena showing some fight to claim a break of her own, Halep responded by breaking again and then served out the match to love.
Arruabarrena won only 37 per cent of points on her first serve and saved just three of eight break points as Halep gave her a lesson and booked a match with compatriot Niculescu.
World number 68 Niculescu had to come through a final-set tie-break to seal her place in the semi-finals as she beat eighth seed Polona Hercog 6-2 2-6 7-6 (7-3).
After the first three games went with serve, Niculescu took the upper hand at 3-1, but Hercog broke back immediately only to then lose her remaining two service games in the set to go 1-0 down.
Hercog regained her composure to hold in all four service games in the second set and broke twice before needing three set points to level the match.
Niculescu was then unable to serve out the match leading 5-3 in the final set as she eventually had to hold her nerve to progress via a tie-break.
Second seed Roberta Vinci will face Kristina Kucova in the other semi-final after the Italian saw off seventh seed Petra Cetkovska 7-5 6-3.
World number 24 Vinci recovered from being broken in the third game but immediately made it 2-2 and then claimed a decisive second break to take the first set.
Cetkovska had no answer to her opponent in the second set as reeled off four games in a row to lead 5-1 and won her fifth match point to go through.
Slovakian Kucova, ranked 166 in the world, was a 6-4 1-6 6-3 winner against Danka Kovinic of Montenegro.