Serena Williams of the US reacts as she plays Ekaterina Makarova of Russia during their semifinals round match on the twelfth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Serena Williams of the US hits a return to Flavia Pennetta of Italy during their quarterfinals round match on the tenth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JASON SZENES

Serena Williams of the US runs towards the net for a return to Flavia Pennetta of Italy during their quarterfinals round match on the tenth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Serena Williams of the US serves to Ekaterina Makarova of Russia during their semifinals round match on the twelfth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Serena Williams of the US hits a return to Flavia Pennetta of Italy during their quarterfinals round match on the tenth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 03 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Serena Williams of the US reacts after defeating Ekaterina Makarova of Russia during their semifinals round match on the twelfth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 05 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JASON SZENES