The tennis star is not vaccinated, he was given an exemption by the tennis association to play without being vaccinated. Australia, like most Western countries, is going full throttle on its covid rules of being vaccinated and those who aren't get sent to a sort of internment camp to quarantine. Since Jokovich managed to be exempted, it is perceived that the Australian immigration were pressed into detaining Jokovich on grounds of his visa, which they cancelled. Jokovich appealed against the cancellation of his visa and won.
Australian government is towing the line of the West, and has arrested Jokovich despite the court ruling in favor of the tennis star. What can possibly be interpreted as making an example of Jokovich, to any other sportsman who resists being vaccinated.