Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: LONDON, England (CNN) -- There is a mathematical reason why the French Open is so different to any other of the Grand Slam tennis tournaments. Clay courts require a different type of game to grass and hard court surfaces The Roland Garros tournament is currently being contested, with Rafael Nadal looking for his fifth win in a row, and world number two Roger Federer searching for his first ever French Open title. • Get the latest French Open news What has brought problems to Federer, and to other greats like Pete Sampras and Boris Becker -- who never won a French Open, is the clay surface on which it is contested.Clay, grass and hard tennis court surfaces all produce different bouncesThe speed and bounce of certain courts can adversely affect a player's gameUse the CNN interactive to see how the bounce differs on each surface 
 Question:One of the problems _ has on clay is that he has a single-handed backhand, and Nadal can get so much spin -- so Federer is trying to hit single-handed shots up around his shoulder."

Roger Federer