2016 Rio Olympics - Tennis training - Olympic Tennis Centre - Rio de Janeiro, Brazil - 04/08/2016. Juan Martin Del Potro (ARG) of Argentina trains. REUTERS/Kevin Lamarque

2016 Rio Olympics - Tennis training - Olympic Tennis Centre - Rio de Janeiro, Brazil - 04/08/2016. Novak Djokovic (SRB) of Serbia trains. REUTERS/Kevin Lamarque

Suspended Minnesota Vikings running back Adrian Peterson exits following his hearing against the NFL over his punishment for child abuse, in New York, New York, U.S. December 2, 2014. REUTERS/Brendan McDermid/File Photo

Maria Verchenova of Russia tees off the 16th hole during the first round of the Evian Masters golf tournament July 21, 2011. REUTERS/Valentin Flauraud

Andy Murray of Britain poses with his two medals during the presentation ceremony for tennis mixed doubles at the All England Lawn Tennis Club during the London 2012 Olympic Games August 5, 2012. REUTERS/Stefan Wermuth

Britain Tennis - Serbia v Great Britain - Davis Cup Quarter Final - Tasmajdan Stadium, Belgrade, Serbia - 16/7/16 Great Britain's Andy Murray Action Images via Reuters / Jason Cairnduff

Arthur Ashe Stadium is seen at sunset ahead of the mens final between Roger Federer of Switzerland and Novak Djokovic of Serbia at the U.S. Open Championships tennis tournament in New York, in this file photo dated September 13, 2015. REUTERS/Lucas Jackson

Fomer Los Angeles Lakers guard Kobe Bryant pauses during a break in playing against the Utah Jazz during Game 4 of their NBA Western Conference semi-final playoff series in Salt Lake City, Utah, in this file May 10, 2010 photo. REUTERS/Lucy Nicholson