Caroline Wozniacki of Denmark hits a return to Shuai Peng of China 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/JUSTIN LANE

Caroline Wozniacki of Denmark reacts as she plays Maria Sharapova of Russia during the seventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Caroline Wozniacki of Denmark reacts after defeating Sara Errani of Italy on the ninth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 02 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JASON SZENES

Caroline Wozniacki of Denmark hits a return to Sara Errani of Italy on the ninth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 02 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JASON SZENES

Caroline Wozniacki of Denmark (R) hits a return to Shuai Peng of China (L) 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/JUSTIN LANE

Caroline Wozniacki of Denmark hits a return to Sara Errani of Italy on the ninth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 02 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JASON SZENES