Angelique Kerber

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Flavia Pennetta reacts during her fourth round match against Angelique Kerber of Germany at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)

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Flavia Pennetta of Italy makes a backhand return to Angelique Kerber of Germany during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Aaron Favila)

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Angelique Kerber of Germany makes a forehand return to Flavia Pennetta of Italy during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)

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Angelique Kerber of Germany reacts between points as she plays Flavia Pennetta of Italy during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)

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Flavia Pennetta of Italy bites the net in frustration during her fourth round match against Angelique Kerber of Germany at the Australian Open tennis championship in Melbourne, Australia, Sunday, Jan. 19, 2014.(AP Photo/Eugene Hoshiko)

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Angelique Kerber, left, of Germany is congratulated by Alison Riske of the U.S. during their third round match at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 17, 2014.(AP Photo/Rick Rycroft)