Serena Williams of the US hits a return to Vitalia Diatchenko of Russia 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

Fabio Fognini of Italy hits a return to Steve Johnson of the US 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/JASON SZENES

Tomas Berdych of the Czech Republic serves to Bjorn Fratangelo of the US 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/JASON SZENES

Milos Raonic of Canada serves to Tim Smyczek of the US 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/JASON SZENES

Simona Halep of Romania hits a return to Marina Erakovic of New Zealand 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