Tennis Player Roger Federer of Switzerland plays tennis for charity on Kids Day at the USTA Billie Jean King National Tennis Center in Flushing Meadows, in New York, New York ,USA, 29 August 2015. EPA/PETER FOLEY

Tennis Player Roger Federer of Switzerland plays for charity on Kids Day at the USTA Billie Jean King National Tennis Center in Flushing Meadows, in New York, New York ,USA, 29 August 2015. EPA/PETER FOLEY

Roger Federer of Switzerland serves to Leonardo Mayer of Argentina during their match on the second day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 01 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Roger Federer of Switzerland hits a return to Leonardo Mayer of Argentina during their match on the second day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 01 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT