World number one Novak Djokovic has pulled out of his Davis Cup quarter-final singles match against Sweden\'s Ervin Eleskovic blaming a knee injury, the news agency TT reported on Friday.The Wimbledon champion was replaced by Janko Tipsarevic for the second rubber of the day.No decision has yet been made regarding Djokovic\'s participation in Sunday\'s singles, when he is scheduled to face Michael Ryderstedt.Serbia came into the quarter-final tie in the southern city of Halmstad as clear favourites to beat their Swedish hosts.The tie opened with a singles rubber between Ryderstedt and Viktor Troicki.