Top seed Angelique Kerber of Germany moved into the quarterfinals at the Citi Open tennis event with a 7-5, 6-0 win over American Melanie Oudin Thursday afternoon.

Kerber's next opponent will be Magdalena Rybarikova, the tournament's defending champion. The seventh-seeded Rybarikova defeated fellow Slovakian Jana Cepelova, 7-5, 5-7, 7-5 on the hardcourts at FitzGerald Tennis Center.

Rybarikova beat Russian Anastasia Pavlyuchenkova in last year's D.C. finale.

In other action involving top-four seeds, No. 3 Russian Ekaterina Makarova drilled France's Caroline Garcia 6-2, 6-0 and No. 4 Alize Cornet of France foiled Brit Heather Watson 6-4, 6-4.

Meanwhile, fifth seed Sorana Cirstea of Romania overcame American Alison Riske 3-6, 6-2, 6-3, but No. 8 seed Madison Keys lost to Romanian Monica Niculescu, 6-1, 2-6, 7-6 (8-6).

In a match involving a pair of non-seeds, Argentine Paula Ormaechea was leading Olga Puchkova 3-6, 6-3, 4-1 when the Russian defaulted from the match. Puchkova was cited for hitting a ball into the knee of a line judge after losing a point.

Ormaechea came into the day with a 2-1 lead in the second set after rain halted play Wednesday.

"It is not the best way to end a match," Ormaechea said via Twitter.

The 2013 Citi Open champ will collect $40,000.