Pironkova cashed in seven of her 12 break point opportunities in a 3-6, 6-4, 6-2 victory over third-seeded Klara Zakopalova. Pironkova saved just one of the seven break points she faced but only one of those came in the third set when she had three breaks of her own.
Kirsten Flipkens beat Bojana Jovanovski 6-3, 6-2 in 59 minutes by winning about 60 percent of the points in the match and wild-card entrant Jarmila Gajdosova stopped Olga Govortsova 6-3, 6-1 with five breaks in eight receiving games in Wednesday's other second-round matches.
Niculescu goes against Flipkens and Pironkova plays ninth-seeded Mona Barthel in the quarterfinals. The other pairings Thursday have Gajdosova meeting Elena Vesnina and eighth-seeded Sloane Stephens squaring off against Lauren Davis.