There is no certainty whether Djokovic will paly in the Aussie Open after his withdrawal from the World Cup series. Djokovic is an anti-vaxxer and since the Aussie Government only grant entry permission to players on condition that they have been vaccinated and tested negative, Djokovic has already withdrawn from the preceding WC series and there has been no indication that he has been vaccinated.
The Mubadala tournament last week - won by Rublev - turned out to be a super-spreader event A string of people - at least six people were subsequently tested positive for Covid 19 and those include Rublev, Nadal and Shapovalov and all is likely to be out of the World Cup series. That series ended on 18 December and theoretically the players can go to Australia by the first week in January 2022. There is no news from Nadal and Shapovalov - but Rublev indicated he had been fully vaccinated and has shown no medical effects stemming from being positive. He is isolating though in terms of the instructions of doctors.
If he is tested negative on Friday he could be in Australia by Sunday - but he will not go if he poses any threat to other people. The absence of players mentioned will weaken the competition - and Rublev in particular will be missed by the Russian team. He easily won all the matches he played in in the 2021 series and helped the Russian team to be the series Champions.