Doubts have emerged over the timing of the positive Covid test Novak Djokovic used to enter Australia earlier this month to compete in the Australian Open.

The test, and the positive result, was seen as an exemption for him from Australia’s strict vaccinated rules.

But it has emerged the serial number on his test on December 16 appears out of sequence with a sample of tests from Serbia over this period.

A German research group Zerforschung has picked up on the discrepancy that the earlier test had a higher confirmation code than the later one.

And the BBC has reported it is also higher than for his second (negative) test result from six days later.

Documents submitted by his lawyers to the Federal Court in Australia included two Covid test certificates, one with a positive result on December 16 and one with a negative result six days later.

The research group told the BBC that all test results have a unique confirmation code and that they wanted to check whether the numbers were generated in strict chronological order.

If they were, the Djokovic numbers would throw up more doubts – why did the earlier test have a higher serial number?

The Zerforschung group collected data from Serbian test results, from a number of legitimate sources, and all showed a chronological order.

The only test that did not conform to the order was Djokovic’s test.

The test on Djokovic was numbered 7371999, which the BBCF reports suggests it fit between test codes for December 25 (7366969) and 28 December (7415312).

That date would make Djokovic ineligible for an exemption.

The BBC conceded in its report that there might have been a glitch, but all the findings have been put to the Serbian health authority and Djokovic’s team, with no response.