China's triple Olympic champion swimmer Sun Yang has told an appeal hearing that he missed a doping test because testers failed to prove their identity.
Sun, 27, was cleared of wrongdoing by Fina, the international swimming federation, in January.
The World Anti-Doping Agency (Wada) is appealing against the decision at a Court of Arbitration for Sport hearing, being held in public in Switzerland.
"I realised they didn't have papers to prove their identification," Sun said.
Speaking via translator during his hour-long evidence at the hearing in Montreux, he added: "The officials were not even capable of proving their identity. How could I allow them to take my sample?
"If they had been professional and had shown their identification, we would not be here today."
It is only the second time such a hearing has been held in public, following a request from multiple world champion Sun.
Technical difficulties with sound and issues with Sun's translator hindered the progress of the hearing on multiple occasions. Ian Meakin, the swimmer's lawyer, complained that the translation standards were "so bad".
"If you want him to answer the question, the translation must be correct," said Meakin, when referring to a Wada question which had been translated into Chinese as "200 millilitres of blood" instead of "200 times".
Sun, who denies smashing with a hammer a vial containing his blood samples, faces a potential eight-year ban if he loses the case.
The freestyle specialist served a three-month doping suspension in 2014 for taking the stimulant trimetazidine, which he says he took to treat a heart condition.
At the World Aquatics Championships in July, Sun won gold in the 200m freestyle but Britain's Duncan Scott refused to share the podium with him.
Australian swimmer Mack Horton took a similar stance after the 400m freestyle competition, years after accusing him of being a "drug cheat".