NUEVA YORK - 24 DE AGOSTO: Seis veces campeón del Grand Slam Novak Djokovic practica para el Abierto de Estados Unidos 2014 en el Billie Jean King National Tennis Center el 24 de agosto de 2014 en Nueva York - Foto e imágenes de stock

NEW YORK - AUGUST 24: Six times Grand Slam champion Novak Djokovic practices for US Open 2014 at Billie Jean King National Tennis Center on August 24 , 2014 in New York

NEW YORK - AUGUST 22, 2018: 13-time Grand Slam champion Novak Djokovic of Serbia practices for the 2018 US Open at Billie Jean King National Tennis

NEW YORK - AUGUST 24: Six times Grand Slam champion Novak Djokovic practices for US Open 2014 at Billie Jean King National Tennis Center on August 24 , 2014 in New York

NEW YORK - AUGUST 24: Six times Grand Slam champion Novak Djokovic practices for US Open 2014 at Billie Jean King National Tennis Center on August 24 , 2014 in New York

NEW YORK - SEPTEMBER 5, 2016: Professional tennis player Ivo Karlovic of Croatia in action during US Open 2016 round four match at Billie Jean King National Tennis Center

NEW YORK - SEPTEMBER 7, 2018: 2018 US Open men' doubles champion Mike Bryan of United States in action during final match at the Billie Jean King National Tennis Center

NEW YORK - AUGUST 24 Professional tennis player David Ferrer practices for US Open 2014 at Billie Jean King National Tennis Center on August 24, 2014 in New York

NEW YORK - AUGUST 21: Six times Grand Slam champion Novak Djokovic practices for US Open 2014 at Billie Jean King National Tennis Center on August 21, 2014 in New York

NEW YORK - AUGUST 21: Six times Grand Slam champion Novak Djokovic practices for US Open 2014 at Billie Jean King National Tennis Center on August 21, 2014 in New York

NEW YORK - AUGUST 25, 2015: Fourteen times Grand Slam Champion Rafael Nadal of Spain practices for US Open 2015 at Billie Jean King National Tennis Center in New York

NEW YORK - AUGUST 23, 2017: Nineteen times Grand Slam Champion Roger Federer of Switzerland practices for US Open 2017 at Billie Jean King National Tennis Center in New York

NEW YORK -SEPTEMBER 2, 2014: Professional tennis player Gilles Simon from France during round 4 match against US Open 2014 champion Marin Cilic at Billie Jean King National Tennis Center in New York

NEW York - AUGUST 25: Professional tennis player Fernando Verdasco practices for US Open 2013 at Billie Jean King National Tennis Center on August 25, 2013 in New York

NEW YORK - AUGUST 22, 2018: 13-time Grand Slam champion Novak Djokovic of Serbia practices for the 2018 US Open at Billie Jean King National Tennis

FLUSHING, NY - AUGUST 24: Professional tennis player David Ferrer practices for US Open 2013 at Billie Jean King National Tennis Center on August 24, 2013 in Flushing, NY

NEW YORK - AUGUST 22, 2018: 13-time Grand Slam champion Novak Djokovic of Serbia practices for the 2018 US Open at Billie Jean King National Tennis

NEW YORK - AUGUST 22, 2018: 13-time Grand Slam champion Novak Djokovic of Serbia practices for the 2018 US Open at Billie Jean King National Tennis

NEW YORK -SEPTEMBER 5, 2015: Seventeen times Grand Slam champion Roger Federer of Switzerland in action during his third round match at US Open 2015 at National Tennis Center in NY

FLUSHING, NY - AUGUST 26 Twelve times Grand Slam champion Rafael Nadal during his first round match at US Open 2013 against Ryan Harrison at Arthur Ashe Stadium on August 26, 2013 in Flushing, NY

NEW YORK - SEPTEMBER 6, 2015: Professional tennis player Jo-Wilfried Tsonga of France in action during his round four match at US Open 2015 at National Tennis Center in New York

NEW YORK - SEPTEMBER 8, 2015: Grand Slam champion Marin Cilic of Croatia in action during his quarterfinal match at US Open 2015 at National Tennis Center in New York

NEW YORK - AUGUST 20: Professional tennis player Robby Ginepri during qualifying match match at US Open 2013 at Billie Jean King National Tennis Center on August 20, 2013 in New York

NEW YORK -SEPTEMBER 2, 2014: Professional tennis player Gilles Simon from France during round 4 match against US Open 2014 champion Marin Cilic at Billie Jean King National Tennis Center in New York

NEW YORK - AUGUST 25: Professional tennis player Novak Djokovic practices for US Open 2013 at Billie Jean King National Tennis Center on August 25, 2013 in New York