Rafael Nadal of Spain celebrates after defeating Roger Federer of Switzerland in their ATP world Tour Finals tennis semifinal match at the O2 Arena on London, Sunday, Nov. 10, 2013. (AP Photo/Alastair Grant)

Photo by: Alastair Grant

Rafael Nadal of Spain celebrates after defeating Roger Federer of Switzerland in their ATP world Tour Finals tennis semifinal match at the O2 Arena on London, Sunday, Nov. 10, 2013. (AP Photo/Alastair Grant)