NEW YORK (Reuters) - Novak Djokovic had hoped to have the crowd behind him as he began his bid to win the U.S. Open and complete the calendar-year Grand Slam, but the Serb said the atmosphere was "not ideal" as Flushing Meadows fans roared on his underdog opponent.

Djokovic, looking to become the first man since Rod Laver in 1969 to win all four slams in the same year, beat Holger Rune 6-1 6-7(5) 6-2 6-1 to advance to the second round after the 18-year-old Danish qualifier ran himself into the ground against the world number one.