Juan Martin del Potro defeats Roger Federer to enter US Open semis

Roger Federer serves in the third set against Del Potro. ( Photo | AP )

Del Potro returns to Roger Federer in the quarterfinals of the U.S. Open tennis tournament in New York. ( Photo | AP )

Roger Federer reaches for a volley during the fourth set of the match. ( Photo | AP )

Del Potro reacts after winning a game against Roger Federer. ( Photo | AP )

Roger Federer reacts after winning a point against Juan Martin del Potro. ( Photo | AP )

Del Potro reacts toward a crowd of supporters after scoring a point Roger Federer. ( Photo | AP )

Roger Federer and Del Potro shake hands after Del Potro won the quarterfinal match at the U.S. Open tennis tournament, Wednesday. ( Photo | AP )

Del Potro reacts after defeating Roger Federer, of Switzerland, during the quarterfinals of the U.S. Open tennis tournament. ( Photo | AP )

Roger Federer acknowledges spectators while leaving the court after losing to Juan Martin del Potro. ( Photo | AP )