Novak Djokovic of Serbia celebrates after defeating Andy Murray of Britain in the men's singles final at the Australian Open tennis championships in Melbourne, Australia, Sunday, Jan. 31, 2016.(AP Photo/Rick Rycroft)
Novak Djokovic of Serbia prepares to make a forehand return to Chung Hyeon of South Korea during their first round match at the Australian Open tennis championships in Melbourne, Australia, Monday, Jan. 18, 2016.(AP Photo/Vincent Thian)
Migrants wait to register with the police in refugee center in the southern Serbian town of Presevo, in Belgrade, Serbia, Friday, Nov. 13, 2015. Hundreds of migrants arrive daily into Serbia in order to register and continue their journey further north towards Western Europe. (AP Photo/Darko Vojinovic)
Prime Minister Viktor Orban has generated headlines with his call for a 13-foot-high fence along Hungary's 110-mile border with Serbia to keep out immigrants seeking to travel from southern Europe to Germany and other wealthy countries. (Associated Press)
Switzerland's Stan Wawrinka celebrates scoring a point in the men's final of the French Open tennis tournament against Serbia's Novak Djokovic at the Roland Garros stadium, in Paris, France, Sunday, June 7, 2015. (AP Photo/David Vincent)
Serbia's Novak Djokovic serves in the second round match of the French Open tennis tournament against Luxembourg's Gilles Muller at the Roland Garros stadium, in Paris, France, Thursday, May 28, 2015. (AP Photo/Francois Mori)
Helena, a transgender person, stands next to her Serbian army uniform put on display in her friend's apartment Wednesday, April 15, 2015, in Belgrade, Serbia. (AP Photo/Marko Drobnjakovic)
Kosovo President Atifete Jahjaga lays flowers on coffins of 21 Kosovo Albanians killed during 1998-99 war with Serbia as they are buried in the village of Cikatove e Vjeter, Kosovo, on Friday, April 17, 2015. (AP Photo/Visar Kryeziu)
Serbia's war crimes prosecutor Vladimir Vukcevic gestures during an interview with The Associated Press in Belgrade, Serbia, Wednesday, March 18, 2015. Prosecutors on Wednesday made Serbia's first arrests of people suspected of carrying out killings in the Srebrenica massacre. It is a milestone in healing the wounds of Europe's worst civilian slaughter since World War II. (AP Photo/Darko Vojinovic)
United States' James Harden throws the ball against Serbia's Nikola Kalinic, left, and Serbia's Milos Teodosic during the final World Basketball match between the United States and Serbia at the Palacio de los Deportes stadium in Madrid, Spain, Sunday, Sept. 14, 2014. (AP Photo/Manu Fernandez)
Kei Nishikori, of Japan, reacts after defeating Novak Djokovic, of Serbia, during the semifinals of the 2014 U.S. Open tennis tournament, Saturday, Sept. 6, 2014, in New York. (AP Photo/Darron Cummings)
Novak Djokovic, of Serbia, reacts after defeating Andy Murray, of Britain, 7-6 (1), 6-7 (1), 6-2, 6-4 during the quarterfinals of the U.S. Open tennis tournament, early Thursday, Sept. 4, 2014, in New York. (AP Photo/John Minchillo)
Ana Ivanovic, of Serbia, returns a shot against Kirolina Pliskova, of the Czech Republic, during the second round of the 2014 U.S. Open tennis tournament, Thursday, Aug. 28, 2014, in New York. (AP Photo/Frank Franklin II)
Ana Ivanovic, of Serbia, returns a shot against Alison Riske, of the United States, during the opening round of the 2014 U.S. Open tennis tournament, Tuesday, Aug. 26, 2014, in New York. (AP Photo/Kathy Willens)