Belgrade, Jan 29 (Reporter) Novak Djokovic has withdrawn from Serbia's Davis Cup tie against Denmark because of a hamstring injury. The 24-time Grand Slam champion was booed off court by some Australian Open fans last week when he retired with the issue after losing the first set of his semi-final against Alexander Zverev, the BBC reported. Djokovic, 37, later posted a photo of an MRI taken of his left hamstring with the caption: "Thought I'd leave this here for all the sports injury 'experts' out there." Serbia are playing Denmark in a first-round qualifying tie, which starts on Friday. "We are weakened by not playing Novak, but we are still going for the win," said Serbia captain Viktor Troicki. "The Danes have a great player in Holger Rune, but our players are also great and I believe in my team...////...