Keys pasted sixth-seeded Mona Barthel 6-3, 6-2 in 59 minutes. Keys lost only 17 points in nine games serving and saved the only break chance Barthel had. She also cashed in three of her five break opportunities.
Rybarikova eliminated No. 1-seeded Kirsten Flipkens 7-6 (7-5), 6-2 while also relying on a strong serving game. She lost her serve once on the day.
Third-seeded Sorana Cirstea took a seesaw 6-1, 1-6, 6-1 decision from 13th-seeded Bojana Jovanovski. Cirstea, who lost her serve twice in the second set, saw only one break point combined in the first and third sets.
No. 5-seeded Sabine Lisicki completed the quarterfinal field with a 7-5, 2-6, 6-4 win over Mirjana Lucic-Baroni. She secured the advance with two third-set breaks.
Also Thursday, Donna Vekic ousted qualifier Maria Sanchez 4-6, 6-2, 6-2; Alison Riske took a 6-1, 6-2 decision over Alla Kudryavtseva in a battle of qualifiers; Daniela Hantuchova took out 12th-seeded Kristina Mladenovic 3-6, 6-6 (8-6), 6-4; and 15th-seeded Francesca Schiavone defeated Marina Erakovic.