Roger Federer enters this year's US Open as the World No.1 with Serbian Novak Djokovic close behind him at No.2. The 17-time Grand Slam champion was ranked No. 1 in the ATP rankings released Monday. Defending champ Novak Djokovic is seeded second and Olympic gold medalist Andy Murray third. Rafael Nadal is out with knee problems giving fellow Spaniard David Ferrer is the No. 4 seed.