Four years on from her only other WTA silverware, on the clay of Bogotá, Arruabarrena mastered the cement - and Niculescu's unorthodoxy - to triumph, 6-0, 2-6, 6-0, in just under two hours.
Arruabarrena came out on the losing side in the pair's only previous hardcourt encounter, last summer in Washington DC. But attacking Niculescu's slice with gusto, she dominated the rematch from the outset.
Yet after racing through the first set the Spaniard handed Niculescu a lifeline, double faulting twice to surrender her serve in the fourth game of the second. It was an opportunity Niculescu grasped with both hands, producing some gloriously inventive tennis - and the odd head-scratching miss - to level the match.
When Arruabarrena then snatched at a routine volley in her opening service game of the decider, the contest looked in danger of slipping away. She found a way out of this hole and when she broke with a wonderful lob a few games later the match swung decisively in her favor.
Niculescu battled to the last, saving one match point. It was merely delaying the inevitable, Arruabarrena wrapping up victory at the second time of asking.