Νέα Υόρκη - 25 Αυγούστου: επαγγελματικού τένις παίκτης Νόβακ Τζόκοβιτς πρακτικές για τις ΗΠΑ ανοικτές 2013 στη Μπίλλι Τζίν Κίνγκ τένις εθνικό κέντρο στις Αυγούστου 25, 2013 στη Νέα Υόρκη - Φωτογραφίες & Εικόνες Αρχείου

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

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

NEW YORK - AUGUST 20: Professional tennis player Mikhail Youzhny practices for US Open 2013 at Louis Armstrong Stadium at Billie Jean King National Tennis Center on August 20, 2013 in New York

KYIV, UKRAINE - JULY 15, 2016: Gerald MELZER of Austria in action during BNP Paribas Davis Cup Europe/Africa Zone Group I game against Illya MARCHENKO of Ukraine at Campa Bucha Tennis Club in Kyiv

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

NEW YORK - AUGUST 29 Professional tennis player Roberto Bautista Agut during second round match at US Open 2013 against David Ferrer at Louis Armstrong Stadium on August 29, 2013 in New York

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

NEW YORK - AUGUST 29 Professional tennis player Roberto Bautista Agut during second round match at US Open 2013 against David Ferrer at Louis Armstrong Stadium on August 29, 2013 in New York

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

FLUSHING, NY - AUGUST 25: Grand Slam champion Andy Roddick practices for US Open at Louis Armstrong Stadium at Billie Jean King National Tennis Center on August 25, 2012 in Flushing, NY.

FLUSHING, NY - AUGUST 23: Grand Slam champion Andy Murray practices for US Open at Louis Armstrong Stadium at Billie Jean King National Tennis Center on August 23, 2012 in Flushing, NY.

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 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

FLUSHING, NY - AUGUST 20: Two times Grand Slam champion Lleyton Hewitt practices for US Open 2013 at Louis Armstrong Stadium at Billie Jean King National Tennis Center on August 20, 2013 in Flushing, NY

NEW YORK -SEPTEMBER 5, 2016: Professional tennis player Kei Nishikori of Japan in action during his round four match at US Open 2016 at Billie Jean King National Tennis Center in New York

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

FLUSHING, NY - AUGUST 25: Grand Slam champion Andy Roddick practices for US Open at Louis Armstrong Stadium at Billie Jean King National Tennis Center on August 25, 2012 in Flushing, NY.

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