New York: Defending champion Novak Djokovic said the timing of the US Open makes it the most unpredictable of the Grand Slams as the Serb made a promising start in his quest for a fourth title here.

The World No. 1, who has won four of the past five majors, brushed past Spain's Roberto Carballes Baena 6-4, 6-1, 6-4 in the opening round Monday. He is attempting to become the first back-to-back men's US Open winner since Roger Federer lifted the trophy five years running from 2004 to 2008.