Rafael Nadal of Spain serves to Borna Coric of Croatia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Rafael Nadal of Spain hits a return to Borna Coric of Croatia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Rafael Nadal of Spain hits a return to Borna Coric of Croatia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Rafael Nadal of Spain hits a return to Borna Coric of Croatia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Rafael Nadal of Spain reacts after defeating Borna Coric of Croatia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO