Roger Federer of Switzerland hits a return to Marin Cilic of Croatia during their semifinals round match on the thirteenth day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 06 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JUSTIN LANE

Angelique Kerber of Germany in action against Timea Babos of Hungary during their fourth round match at the Miami Open tennis tournament on Key Biscayne, Miami, Florida, USA, 28 March 2016. EPA/RHONA WISE

Dominic Thiem of Austria in action against Nicolas Almagro of Spain during their second round match at the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 20 January 2016. EPA/LYNN BO BO

Richard Gasquet from France in action against Marin Cilic from Croatia during their fourth round match at the BNP Paribas Open tennis tournament in Indian Wells, California, USA, 16 March 2016. EPA/MIKE NELSON

Rafael Nadal from Spain in action against Alexander Zverev from Germany during their fourth round match at the BNP Paribas Open tennis tournament in Indian Wells, California, USA, 16 March 2016. EPA/MIKE NELSON