Angelique Kerber of Germany returns a ball to Ekaterina Makarova of Russia during their quarterfinal match at the WTA tennis tournament in Stuttgart, Germany, 24 April 2015. EPA/MARIJAN MURAT

Japanese player Kei Nishikori returns the ball to Spaniard Roberto Bautista during their quarterfinal match for the Conde Godo tennis tournament, in Barcelona, northeastern Spain, 24 April 2015. EPA/TONI ALBIR

Denmark’s Caroline Wozniacki returns a ball to Spain’s Carla Suarez Navarro during their quarterfinal match for the WTA tennis tournament in Stuttgart, Germany, 24 April 2015. EPA/MARIJAN MURAT

Rafael Nadal of Spain reacts after losing his third round match to Fabio Fognini of Italy at the Conde de Godo tennis tournament at the Real Club de Tenis in Barcelona, Spain, 23 April 2015. EPA/ALEJANDRO GARCIA

Maria Sharapova of Russia serves a ball to Angelique Kerber of Germany during their second round match at the WTA tennis tournament in Stuttgart, Germany, 23 April 2015. EPA/MARIJAN MURAT

David Ferrer of Spain returns a ball to Philipp Kohlschreiber of Germany during their quarterfinals match at the Conde de Godo tennis tournament at the Real Club de Tenis in Barcelona, Spain, 24 April 2015. EPA/TONI ALBIR