MELBOURNE: Defending champion Novak Djokovic issued a chilling warning to his Australian Open challengers on Tuesday by storming to a 6-3 6-2 6-4 victory over seventh seed Kei Nishikori to set up a blockbuster semi-final against Roger Federer. The world number one was far from his imperious best and struggled on serve but was content to allow his Japanese opponent to fall on his own sword with 54 unforced errors under the Rod Laver Arena floodlights. Nishikori broke the Serb twice in the third set and edged 3-1 ahead but was unable to capitalise on his own serve and threw his racquet onto the court in a rare show of frustration. Djokovic fired a sizzling cross-court backhand winner to close out the match and will make his sixth appearance in the last four at Melbourne Park, having avenged his loss to Nishikori in the 2014 US Open semi-finals.