German Petkovic, the ninth seed in Beijing, set up a final clash with Poland's Agnieszka Radwanska by posting a 6-2 6-0 victory against world number 57 Niculescu.
The 24-year-old Romanian was appearing in her third WTA semi-final but the most prestigious so far, and had claimed the scalps of Li Na and Maria Kirilenko on her way through to the last four. However she was overwhelmed by Petkovic, who dropped just nine points in the second set.
Italian Flavia Pennetta produced a stunning result yesterday by eliminating world number one Caroline Wozniacki, but she could not follow it up with another success against a seed in the semi-finals.
Pennetta was turned over 6-2 6-4 by 11th seed Radwanska.