LONDON, ENGLAND - JULY 01:  Serena Williams of the United States struggles during the warm up prior her Ladies Doubles second round match with Venus against Kristina Barrois of Germany and Stefanie Voegele of Switzerland on day eight of the Wimbledon Lawn Tennis Championships at the All England Lawn Tennis and Croquet Club on July 1, 2014 in London, England.  (Photo by Jan Kruger/Getty Images)