David Ferrer of Spain hits a return to Jeremy Chardy of France during their match on the fifth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Marin Cilic of Croatia signs autographs after defeating Mikhail Kukushkin of Kazakhstan during their match on the fifth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Eugenie Bouchard of Canada hits a return to Dominka Cibulkova of Slovakia during their match on the fifth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Novak Djokovic of Serbia hits a return to Andreas Seppi of Italy during their match on the fifth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Bethanie Mattek-Sands of the US reacts as she plays Serena Williams of the US during their match on the fifth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO