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Germany's Mischa Zverev (L) talks to Switzerland's Roger Federer after Federer won their men's singles quarter-final match on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read PAUL CROCK/AFP/Getty Images)

Germany's Mischa Zverev (L) talks to Switzerland's Roger Federer after Federer won their men's singles quarter-final match on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read PAUL CROCK/AFP/Getty Images)

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Former tennis player Jim Courier (L) interviews Switzerland's Roger Federer after he won his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / GREG WOOD / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read GREG WOOD/AFP/Getty Images)

Former tennis player Jim Courier (L) interviews Switzerland's Roger Federer after he won his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / GREG WOOD / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read GREG WOOD/AFP/Getty Images)

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A supporter of Switzerland's Roger Federer holds a sign during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read PAUL CROCK/AFP/Getty Images)

A supporter of Switzerland's Roger Federer holds a sign during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read PAUL CROCK/AFP/Getty Images)

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Fans of Switzerland's Roger Federer hold the Swiss national flag during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read PAUL CROCK/AFP/Getty Images)

Fans of Switzerland's Roger Federer hold the Swiss national flag during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read PAUL CROCK/AFP/Getty Images)

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A supporter of Switzerland's Roger Federer holds a sign during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read PAUL CROCK/AFP/Getty Images)

A supporter of Switzerland's Roger Federer holds a sign during his men's singles quarter-final match against Germany's Mischa Zverev on day nine of the Australian Open tennis tournament in Melbourne on January 24, 2017. / AFP / PAUL CROCK / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read PAUL CROCK/AFP/Getty Images)