Two-time Grand Slam champion Garbine Muguruza lost her opening match of the North American hardcourt season on Wednesday (Aug 15), falling 2-6, 6-4, 6-4 to Lesia Tsurenko in Cincinnati.
Muguruza, competing in her first tune-up event for the August 27-Sept.
Second seed Caroline Wozniacki, however, retired with a left knee injury after surrendering the first set 6-4 to Kiki Bertens.
Tsurenko stormed back to go ahead 5-4 and sealed the contest on her first match point when a Muguruza forehand from behind the baseline sailed long, giving the Ukrainian world number 44 her first top-10 victory since 2015.
Tsurenko took down the seventh-seeded Spaniard 2-6 6-4 6-4 in the early match, which was a preview of the challenges faced by top seeds during the day. Danish star Wozniacki had just lost the opening set 6-4 when she withdrew in the second-round clash, which was delayed by rain for more than three hours.
For a place in the quarter-finals of the Cincinnati Lesia, Tsurenko will compete with the Russian, Ekaterina Makarova. "I think that that was really an important match for me, to get my rhythm, feel the ball, play tough and close matches again, especially now after few weeks off", Kerber said.More news: Death toll rises to 42 in Italy bridge collapse ahead of funerals
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"I'm sorry that she had to retire", Bertens said.
"To believe that I don't have to be 100% ready or in the best moment of your life to win it". Just experience to work hard and to never lose the, how you say, motivation to believe you can do it'.
Also advancing to the third round was Belarusian Aryna Sabalenka, who upset Czech ninth seed Karolina Pliskova 2-6 6-3 7-5, and Australian Ashleigh Barty, a 7-5 6-3 victor over Estonia's Kaia Kanepi.