Serbia

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Novak Djokovic of Serbia reacts after he missed a ball against Roger Federer of Switzerland during a semi final match of the Dubai Tennis Championships in Dubai, United Arab Emirates, Friday, Feb. 28, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Roger Federer of Switzerland during a semi final match of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Friday, Feb. 28, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Roger Federer of Switzerland during a semi final match of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Friday, Feb. 28, 2014. (AP Photo/Kamran Jebreili)

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Roger Federer of Switzerland returns the ball to Novak Djokovic of Serbia during a semi final match of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Friday, Feb. 28, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Spain's Roberto Bautista Agut during the third day of the Dubai Tennis Championships in Dubai, United Arab Emirates, Wednesday, Feb. 26, 2014. (AP Photo/Kamran Jebreili)

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Spain's Roberto Bautista-Agut returns the ball to Novak Djokovic of Serbia during the third day of the Dubai Tennis Championships in Dubai, United Arab Emirates, Wednesday, Feb. 26, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Spain's Roberto Bautista-Agut during the third day of the Dubai Tennis Championships in Dubai, United Arab Emirates, Wednesday, Feb. 26, 2014. (AP Photo/Kamran Jebreili)

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Spain's Roberto Bautista-Agut returns the ball to Novak Djokovic of Serbia during the third day of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Wednesday, Feb. 26, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia serves the ball to Denis Istomin of Uzbekistan during the second round of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Tuesday, Feb. 25, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Denis Istomin of Uzbekistan during the second round of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Tuesday, Feb. 25, 2014. (AP Photo/Kamran Jebreili)

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Novak Djokovic of Serbia returns the ball to Denis Istomin of Uzbekistan during the second round of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Tuesday, Feb. 25, 2014. (AP Photo/Kamran Jebreili)