SYDNEY: Nick Kyrgios Tuesday pulled out of a World Tennis Challenge exhibition in Adelaide with a knee injury in a setback to his Australian Open preparations. “Unfortunately I am unable to play @WTCAdelaide this year. I have to rest & do all I can to be ready for #AO17 but I will be back next year,” the Australian world number 14 said on Twitter. The 21-year-old is receiving treatment on a left knee he reportedly damaged playing basketball, and will rest in the lead-up to the opening Grand Slam of the year starting on Monday in Melbourne. He struggled with the issue during a 6-2, 6-2 defeat in less than an hour to American Jack Sock at last week’s Hopman Cup. Kyrgios appeared less hampered on Monday night when he beat Rafael Nadal at an exhibition in Sydney under the Fast4 format. Kyrgios’s career has been marked by tantrums and even tanking, with Australia’s highest ranked player only recently returning from a ban for a “lack of best efforts” during a controversial match in Shanghai last year.