NEW YORK (Reuters) - Novak Djokovic took a first unsteady step toward completing the calendar-year Grand Slam on Tuesday as the Serb laboured into the second round of the U.S. Open with a 6-1 6-7(5) 6-2 6-1 win over dashing Danish teenager Holger Rune.

On paper, the contest was a total mismatch - an 18-year-old qualifier making his Grand Slam debut against a player chasing a 21st major title.