MADRID - Top-seeded Agnieszka Radwanska and Australian Open champion Angelique Kerber were upset in the opening round of the Madrid Masters on Sunday.

Radwanska, who was elevated to the No. 1 seeding when Serena Williams withdrew, was beaten 4-6, 7-6 (3), 3-6 by Dominika Cibulkova of Slovakia.

Radwanska hit five aces but also had six double-faults and was broken 11 times by Cibulkova.

Barbora Strycova of the Czech Republic beat second-seeded Kerber 6-4, 6-2.

Cibulkova was coming off a win at the Katowice Open last month and a narrow loss to Radwanska at Indian Wells, where she had a match point.

Cibulkova will next meet Caroline Garcia, who was leading 6-4, 2-1 when Johanna Konta retired from their match.

Heading into their clay-court opener, Kerber hadn't dropped a single set to Strycova in four prior meetings.

But the third-ranked Kerber was broken by Strycova four times to fall in a little over an hour.

The 33rd-ranked Strycova faces Madison Keys, who rallied past fellow American Alison Riske 1-6, 6-3, 6-4.

Third-seeded Garbine Muguruza had little trouble advancing past Anna Karolina Schmiedlova 6-2, 7-5, breaking her serve six times.

"In Madrid we have this extra pressure because I'm here at home," Muguruza said. "Perhaps you're a little bit more nervous because of that. But my first feeling is that I'm satisfied for winning the match, fighting, as I fought a lot."

Defending champion Petra Kvitova, seeded fifth, eased past Lara Arruabarrena 6-3, 6-2, and fourth-seeded Victoria Azarenka defeated Laura Robson 6-4, 6-2.

Azarenka, in her first tournament since winning both Indian Wells and Miami, improved her record to 25 wins and one loss this year.

Sixth-seeded Simona Halep downed Misaki Doi 6-0, 6-3, and eighth-seeded Carla Suarez Navarro had a 3-6, 6-2, 6-3 win over Timea Babos.

Also advancing were 11th-seeded Lucie Safarova, 13th-seeded Karolina Pliskova, 14th-seeded Ana Ivanovic, Samantha Stosur and Sabine Lisicki.

Other seeded players making early exits were No. 7 Roberta Vinci and No. 9 Svetlana Kuznetsova.

In the first round of the men's tournament, Fernando Verdasco beat Juan Monaco 7-5, 7-6 (3), and Alexandr Dolgopolov outlasted Steve Johnson 3-6, 6-3, 7-6 (4).