Some fantastic sleuthing by Der Spiegel and a German research group called Zerforschung, highlighted here by the BBC:

“Doubts have emerged over the timing of the positive Covid test Novak Djokovic used to enter Australia to try to compete in the Australian Open. It was provided to exempt him from rules barring unvaccinated people.

“However, the serial number on his test on 16 December appears out of sequence with a sample of tests from Serbia over this period gathered by the BBC.”

It’s a great example of what’s known in statistics as the German tank problem, after the most famous instance of it in World War II.