Wozniacki, the No. 1 for the tournament, defeated Niculescu 6-3, 6-2. She won half the points -- 39-of-78 -- when Niculescu served and saved all but one of the four break points she faced.
Second-seeded Sloane Stephens and No. 3 Sabine Lisicki also moved into the quarterfinals after winning in straight sets. Stephens topped Yvonne Meusburger 7-5, 6-4 with six breaks, while Lisicki was a quick 6-4, 6-0 victory over Tereza Smitkova. Lisicki lost only 14 points over her eight games serving Thursday.
Wozniacki faces eighth-seeded Bojana Jovanovski and Lisicki plays Knapp in the quarterfinals of the top half of the draw. Stephens meets Voegele and Annika Beck plays qualifier Katarzyna Piter in the other Friday matches.
The formal name of the tournament is the BGL BNP Paribas Luxembourg Open.