Serena Williams of the US hits a return to Roberta Vinci of Italy during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Roberta Vinci of Italy hits a return to Serena Williams of the US during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 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 Roberta Vinci of Italy during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 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 Roberta Vinci of Italy during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES

Roberta Vinci of Italy reacts after taking the second set Serena Williams of the US during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO