Sharapova – making her first appearance in Qatar since 2013 – came into the tournament with a fine record in Doha, the Russian having made at least the semi-finals in each of her three previous appearances.
She was champion in 2005 and 2008, and was only denied five years ago by Serena Williams in the last four.
Keeping up that impressive run proved a step too far against Niculescu, though, despite a strong start to their match.
Sharapova broke three times in the opening set to move ahead, however she struggled to confirm the 1-0 lead, needing nine break chances to move 5-2 ahead.
The wildcard then failed to serve out for the set, wasting four opportunities before eventually getting the job done.
It looked like being a straightforward victory from there but Niculescu refused to give in, battling away from the baseline and ruthlessly taking her chances when errors crept into Sharapova's game.
Niculescu's fondness for the slice caused her illustrious opponent problems, Sharapova allowing the match to slip from her grasp and her body language changing as the contest entered a deciding set.
The world number 41's frustrations became all too apparent in the third as she tried to force the play, but her timing was off and she finished the match with 52 unforced errors.
Niculescu broke in the eighth game of the final set to open the door, and more mistakes from Sharapova allowed the Romanian to serve out to love to progress to round two.