Serena Williams of the US serves to her sister Venus Williams in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN

Venus Williams of the US returns to her sister Serena Williams in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN

Serena Williams of the US returns to her sister Venus Williams in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN

Venus Williams of the US plays her sister Serena Williams in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN

Serena Williams of the US (L) at the net with her sister Venus Williams whom she defeated in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN

Serena Williams of the US (R) walks off court with her sister Venus Williams whom she defeated in their fourth round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 06 July 2015. EPA/ANDY RAIN