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Serbia's Novak Djokovic stretches as play is halted while the roof is closed during his men's singles semi-final match against Switzerland's Roger Federer on day eleven of the 2016 Australian Open tennis tournament in Melbourne on January 28, 2016. AFP PHOTO / SAEED KHAN- / AFP / SAEED KHAN (Photo credit should read SAEED KHAN/AFP/Getty Images)

Serbia's Novak Djokovic stretches as play is halted while the roof is closed during his men's singles semi-final match against Switzerland's Roger Federer on day eleven of the 2016 Australian Open tennis tournament in Melbourne on January 28, 2016. AFP PHOTO / SAEED KHAN- / AFP / SAEED KHAN        (Photo credit should read SAEED KHAN/AFP/Getty Images)