Caroline Wozniacki of Denmark hits a return to Samantha Stosur of Australia during a women’s singles match in the China Open tennis tournament at the National Tennis Center in Beijing, China 01 October 2014. EPA/ROLEX DELA PENA

Kei Nishikori of Japan serves to Ivan Dodig of Croatia during their singles first round match at the Japan Open Tennis Championships in Tokyo, Japan, 01 October 2014. EPA/KIYOSHI OTA

Serena Williams of the USA in action against Tsevatana Pironkova of Bulgaria during their second round match at the Tennis China Open in Beijing, China, 30 September 2014. EPA/DIEGO AZUBEL

Spain’s Rafael Nadal in action against France’s Richard Gasquet during their first round match at the Tennis China Open in Beijing, China, 30 September 2014. EPA/DIEGO AZUBEL

Chinese tennis player Li Na holds a framed picture during her retirement ceremony on the sidelines of the Tennis China Open in Beijing, China, 30 September 2014. China’s two-time grand slam champion Li Na announced her retirement from the sport due to ‘chronic injuries’. The 32-year-old, who broke all barriers for women in Chinese sport and earned a world-class endorsement fortune in the process, had not played since Wimbledon due to a knee injury. EPA/DIEGO AZUBEL

Britain’s Andy Murray reacts during his first round match against Poland’s Jerzy Janowicz at the Tennis China Open in Beijing, China, 30 September 2014. EPA/DIEGO AZUBEL