The unseeded Williams -- who last won a singles title in February 2010 -- gained the championship match with a 5-7, 6-4, 6-4 decision over Germany's Andrea Petkovic.
Niculescu breezed into the final with a 6-1, 6-3 win over Slovakian Daniela Hantuchova.
Williams will be seeking her 43rd career title while Niculescu will be making only her second finals appearance -- she lost to Victoria Azarenka in the Luxembourg title tilt last year.
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