Serena Williams of the US chases a return to Venus Williams of the US during their quarterfinals match on the ninth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Venus Williams of the US hits a return to Serena Williams of the US during their quarterfinals match on the ninth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Venus Williams of the US hits a return to Serena Williams of the US during their quarterfinals match on the ninth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES

Serena Williams of the US hits a return to Venus Williams of the US during their quarterfinals match on the ninth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/DANIEL MURPHY

Venus Williams of the US hits a return to Serena Williams of the US during their quarterfinals match on the ninth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 08 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JUSTIN LANE