MASON, Ohio (AP) — Victoria Azarenka reached the quarterfinals by defeating No. 33 Magdalena Rybarikova 6-3, 6-4 Thursday at the Western & Southern Open.

The two-time Australian Open champion and her Slovakian opponent had 13 service breaks, including the last seven games.

Seventh-seeded Juan Martin del Potro earned a berth in the men's quarterfinals, holding off 27th-ranked Spaniard Feliciano Lopez for a 6-4, 1-6, 6-4 win.

Azarenka withdrew from last week's Rogers Cup in Toronto with a lower back injury. The previous week, she lost to Samantha Stosur in the finals of the Southern California Open.

Azarenka reached the quarterfinals for the first time in three appearances at the event. She lost to Jelana Jankovic in the third round last year.