Novak Djokovic, of Serbia, reacts after defeating Jan-Lennard Struff, of Germany, during the fourth round of the US Open tennis championships in New York.

Novak Djokovic, of Serbia, reacts after defeating Jan-Lennard Struff, of Germany, during the fourth round of the US Open tennis championships in New York.

Jan-Lennard Struff, of Germany, returns a shot to Novak Djokovic, of Serbia, during the fourth round of the US Open tennis championships in New York.

Novak Djokovic, of Serbia, returns a shot to Jan-Lennard Struff, of Germany, during the fourth round of the US Open tennis championships in New York.

Jan-Lennard Struff, of Germany, serves to Novak Djokovic, of Serbia, during the fourth round of the US Open tennis championships, in New York.

Novak Djokovic, of Serbia, chases down a shot form Jan-Lennard Struff, of Germany, during the fourth round of the US Open tennis championships in New York.

Jan-Lennard Struff, of Germany, returns a shot to Novak Djokovic, of Serbia, during the fourth round of the US Open tennis championships in New York.

Novak Djokovic, of Serbia, returns a shot to Jan-Lennard Struff, of Germany, during the fourth round of the US Open tennis championships in New York.