Novak Djokovic plays a forehand in his fourth round match against Stanislas Wawrinka during day seven of the Australian Open. Photo by Julian Finney/Getty Images
Na Li serves in her fourth round match against Julia Goerges. Photo by Ryan Pierse/Getty Images
Julia Goerges of Germany signs an autograph after losing her fourth round match against Na Li. Photo by Scott Barbour/Getty Images
Agnieszka Radwanska shakes hands with Ana Ivanovic after their fourth round match. Photo by Julian Finney/Getty Images
Detail of official tennis balls during day seven of the 2013 Australian Open. Photo by Vince Caligiuri/Getty Images
Stanislas Wawrinka plays a backhand in his fourth round match against Novak Djokovic. Photo by Robert Prezioso/Getty Images
Ana Ivanovic plays a forehand in her fourth round match against Agnieszka Radwanska. Photo: Robert Prezioso/Getty Images
Agnieszka Radwanska plays a backhand in her fourth round match against Ana Ivanovic. Photo by Julian Finney/Getty Images
A general view of Rod Laver Arena during day seven of the Australian Open. Photo by Julian Finney/Getty Images
Novak Djokovic plays a backhand in his fourth round match. Photo: Robert Prezioso/Getty Images
