Second-seeded Sloane Stephens and top-seeded Angelique Kerber reached the quarterfinals of the Generali Ladies in contrasting styles on Thursday.Stephens needed 2 1/2 hours to defeat Andrea Petkovic of Germany 7-6 (1), 4-6, 6-3, and Kerber cruised past Alexandra Cadantu of Romania 6-1, 6-0 in just 50 minutes.The 12th-ranked Stephens won just two of the first 14 points and fell behind 5-2 in the first set against Petkovic, but the American saved three set points before dominating in a tiebreaker."I let out some screams, which I normally don't do," Stephens said about her slow start. "I am just happy to get through."Stephens went 3-1 up in the second set before Petkovic reeled off 10 straight points to turn it around. She managed an early break in the third and the Australian Open semifinalist closed out the victory on her third match point.Stephens will play Stefanie Voegele of Switzerland, who rallied to defeat Karin Knapp of Italy 4-6, 6-2, 7-5 to advance to her first quarterfinals since April.Kerber was broken while serving for the first set but otherwise cruised past the 72nd-ranked Cadantu, who scored just eight points in total on her own serve."I was very relaxed and played well from the start," said the 10th-ranked Kerber.Kerber is one win short of securing the eighth and final spot at the season-ending WTA Championships in Istanbul. She takes on Patricia Mayr-Achleitner in the quarterfinals. The Austrian lost two match points at 5-3 in the second set before defeating Maryna Zanevska of Ukraine 6-1, 6-7 (1), 6-4.Earlier, Dominika Cibulkova of Slovakia routed Katarzyna Piter of Poland 6-0, 6-1 in less than 53 minutes. Piter, playing in her first main draw of the year, broke the seventh-seeded Cibulkova in the second set for her only game.In her fourth quarterfinal of the season, Cibulkova will take on two-time former champion Ana Ivanovic.