World tennis number one Novak Djokovic weathered a first-set storm to subdue the Ukranian Alexandr Dolgopolov in straight sets and reach the quarter-finals of the US Open in New York.
Djokovic set up an all-Serbian last-eight clash with Davis Cup teammate Janko Tipsarevic, but first he had to quell the challenge presented by the world number 23.
Djokovic, who has lost just two matches this year and claimed the Australian Open and Wimbledon titles as he surged to the top of the rankings, needed six set points -- and had to save four -- to claim the first-set tiebreaker 16-14.
