Top-seeded Lucie Safarova of the Czech Republic withdrew with a left thigh muscle injury before her second round match against Sweden's Johanna Larsson at the Prague Open on Wednesday.
"It was painful even before the tournament but it got worse after the first round," said Safarova, who beat Anastasija Sevastova of Latvia 6-4 7-5 in the opening round on Tuesday.
Safarova will rest until next Friday when the result of a check-up determines whether she will be able to join the hard court season in the United States.
Larsson will now face eighth-seeded Czech player Barbora Zahlavova Strycova, who overcame Ksenia Pervak of Russia 2-6 6-4 6-2.
Also Anna Tatishvili of Georgia advanced to the quarter-finals by beating Eva Hrdinova of the Czech Republic 6-4 6-3.
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