Doubts have been raised regarding the timing of Novak Djokovic’s Covid tests amid suggestions that the world number one lied about when he tested positive for Covid in December.
According to BBC, it was provided to exempt him from rules barring unvaccinated people.
However, the serial number on his test on December 16 appears out of sequence with a sample of tests from Serbia over this period gathered by the BBC.
It is also higher than for his second (negative) test result from six days later.
His visa application was ultimately rejected, but not on these grounds.
These findings raise questions over what impact a later positive test result would have had on his ability to enter Australia.
What are people questioning?
Documents submitted by his lawyers to federal court in Australia included two Covid (PCR) test certificates, one with a positive result on December 16 and one with a negative result on December 22.
A German research group called Zerforschung first picked up on the discrepancy that the earlier test had a higher confirmation code than the later one.
They wrote a blog titled "Novak Djokovic's time-travelling PCR tests", and partnered with German news site Der Spiegel who reported on the issue.
Documents submitted to the federal court in Australia included one from the acting director of Serbia's official health body, confirming the dates on these certificates accurately reflected when the tests had been carried out.
Digging deeper into the numbers
Mr Djokovic was tested in Serbia, and received his results from the Institute of Public Health of Serbia.
All these test results have a unique confirmation code.
We wanted to check whether these numbers are generated in strict chronological order in a single national database at the time of processing.
If so, it would bring into question why the earlier test had a higher serial number.
So we collected data from as many Serbian test certificates as we could to plot these confirmation codes on a timeline.
Each dot on the graph above represents a single confirmation code on a test certificate we obtained.
Of these, 21 were provided by BBC colleagues based in Serbia, all issued by the Institute of Public Health.
A further 35 were obtained from Milovan Suvakov, a Serbian research scientist based in the US, who had been posting some of his own data on Twitter.