In an anonymous interview with Chinese state news agency Xinhua, the man said he had never been trained in doping control and claimed he knew nothing about the test or his role on the night which could cost Sun his career.

He claimed he had provided written testimony to the World Anti-Doping Agency (WADA) and the Court of Arbitration for Sport (CAS) several days before Sun's public hearing in Montreux last Friday (November 15).

In the latest twist to a case which could see Sun banned from Tokyo 2020, the man also insisted he had offered to tell his story via video prior to the hearing but his request was never followed up.

"I am a builder and I am always busy at work, day and night," he told Xinhua.

"No one ever trained me about the doping test, and it is unnecessary for me to undertake such training."

At the CAS hearing, the first to be held in public for 20 years, Sun and his legal team argued the testers who arrived at his home in Zheijang province on September 4 last year were not properly accredited.

WADA, challenging the decision from the International Swimming Federation (FINA) to let Sun off with a warning despite a member of his entourage smashing his blood vial with a hammer, disputed this and said the credentials of the officials were in order.

In its ruling, the FINA doping panel cited the "extremely unprofessional" conduct of a chaperone, who took photos and video of the 11-time world champion on his phone during the ill-fated test.

The unidentified builder admitted the claims in an earlier interview with Xinhua, where he said he had been asked to help with the test by the doping control officer who happened to be a former classmate.

"Sun is a big star in China and it was my first time being near him," he said. 

"I was excited. 

"I took a couple of pictures outside the room with my cellphone. 

"When I tried to take pictures of him again when we were sitting in the room, Sun told me not to do so.

"Then he asked every one of us to identify ourselves. 

"I showed my ID card. 

"Sun pointed out that I was not an accredited tester and should not stay in the test room."

The lack of accreditation sparked the row which ended up in a blood vial given by Sun being destroyed.

Lawyers for WADA, seeking to ban Sun for up to eight years, argued at the hearing that the 27-year-old was guilty of tampering before the vial was smashed.

WADA appealed as it believed FINA had been too lenient in issuing Sun with a warning.

Sun, winner of the 400 and 1,500 metres freestyle gold medals at the 2012 Olympic Games in London and the 200m freestyle in Rio de Janeiro four years later, is facing a stricter punishment due to an earlier suspension for a banned substance in 2014.

A ruling from CAS could come in January, depending on how long it takes to translate Sun's opening and closing statements following issues with the simultaneous interpretation during the hearing.

insidethegames has contacted WADA and FINA for comment.