Venus Williams of the USA in action against Elena Vesnina of Russia during their match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 25 March 2016. EPA/RHONA WISE

Serena Williams of the USA in action against Christina Mchale of the USA during their match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 24 March 2016. EPA/RHONA WISE

Venus Williams of the US in action against Kurumi Nara of Japan during their second round match at the BNP Paribas Open in Indian Wells, California, USA, 11 March 2016. EPA/EUGENE GARCIA

Serena Williams of USA in action against Yulia Putintseva of Kazahkstan during their third round match at the BNP Paribas Open tennis tournament in Indian Wells, California, USA, 13 March 2016. EPA/MIKE NELSON

Serena Williams of the United States reacts after a shot against Angelique Kerber of Germany during the Womens final match at the Australian Open tennis tournament in Melbourne, Australia, 30 January 2016. EPA/TRACEY NEARMY

Venus Williams of the US (L) and Serena Williams of the US (R) embrace at the net after 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