Poland's Radwanska, the number one seed in the absence of Serena Williams, lost 6-4, 6-7 (3/7), 6-3 to the rejuvenated Dominika Cibulkova of Slovakia.
Radwanska, a semi-finalist in 2012 and 2014 in the Spanish capital, already knew the dangers posed by Cibulkova after her opponent came within a point of victory in their last meeting at Indian Wells in March.

Cibulkova, a former world number 10 and Australian Open runner-up, was always going to be a dangerous floater having won her fifth career title last time out in Katowice three weeks ago.

Second-seeded German Kerber was dumped out by the Czech Republic's Barbora Strycova 6-4, 6-2 with her defeat coming just a week after she had retained her title in Stuttgart. However, Kerber was outplayed by world number 33 Strycova, who she had beaten 6-1, 6-1 in Miami back in March.

"I'd win four games in the matches we played before," said Strycova.

"I went into this match thinking I had nothing to lose. I mean, it can't be worse than Miami."

Losses for Radwanska and Kerber mean Garbine Muguruza is the highest seed left at the key clay-court event and the fourth-ranked Spaniard sealed her place in round two with a 6-2, 7-5 win over Anna Schmiedlova.

"Of course I was a little bit nervous and tense because the first matches are always complicated," said Muguruza, who is aiming to be the first Spanish woman to win the title in Madrid.

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