Melbourne, Dec 26 : World No. 1 tennis player Novak Djokovic has added further mystery to his participation in next year's Australian Open -- where he is the defending champion --, reportedly opting to skip the ATP Cup, scheduled in Sydney from January 1-9. According to Serbian newspaper Blic, a member of his team reportedly said, "It is 99 per cent sure that Novak won't go to the ATP Cup. He is training here (in Belgrade) but he has decided to give that tournament a miss." The 34-year-old, who ended the 2021 season as world No.1 for a record seventh time to break the tie with American great Pete Sampras, has kept the organisers guessing about his COVID-19 vaccination status and his reported pull-out of the multi-nation ATP Cup has made his Australian Open participation more complex. Djokovic is opposed to the Australian Open organisers' demand to allow only fully-vaccinated individuals compete at Melbourne Park. While tournament director Craig Tiley has said that a very small percentage of players will be granted medical exemption, he has not said whether the nine-time Australian Open champion will get it. The Australian Open commences on January 17 and the Victorian state-enforced COVID-19 vaccine mandate has put pressure on Djokovic's availability, who has refused to reveal his vaccination status. The Serbian has stated that his vaccination status is a "personal" matter. Djokovic's father, Srdjan, dropped the biggest hint yet that the world No.1 hasn't taken the vaccine when he told Serbian TV that, "They have to decide whether they will let Novak in. "If they decide (not to let him in), they have to stand behind their decision. He has won the Australian Open nine times, he now wants to play -- but they have to permit him to play. Novak will do what he thinks is the best for him, nobody will impose an opinion on him," Wide World of Sports quoted Srdjan as saying on Serbian TV. /IANS