Serena Williams of the US serves to Bethanie Mattek-Sands 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.

Bethanie Mattek-Sands of the US hits a return to 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

Serena Williams of the US hits a return to Bethanie Mattek-Sands 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

Bethanie Mattek-Sands of the US hits a return to 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

Serena Williams of the US reacts as she plays Bethanie Mattek-Sands 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