It has emerged the Serbian tennis player and his wife hold a majority stake in a Danish biotech firm working to develop a treatment for coronavirus.

Ivan Loncarevic, the CEO of QuantBioRes, told Reuters the 34-year-old acquired the 80% stake in June 2020. He declined to say how much it cost.

QuantBioRes is developing a peptide, which inhibits the coronavirus from infecting a human cell.

It expects to launch clinical trials in the UK this summer, Mr Loncarevic said. He stressed the firm was working on a treatment, not a vaccine.

Mr Loncarevic said the company has around a dozen researchers spread across Denmark, Australia and Slovenia.

Djokovic and his wife, Jelena, own 40.8% and 32.9% of the company respectively, according to the Danish company register.

The world tennis number one returned to the Serbian capital Belgrade on Monday after he was deported from Australia following the decision of Immigration Minister Alex Hawke to cancel his visa.

A small crowd of supporters waited for him outside the terminal at Nikola Tesla Airport

He had hoped to win his 21st grand slam title, which would place him above rivals Roger Federer and Rafa Nadal.

The cancellation of his visa on public interest grounds was upheld by Australian judges.

His visa was revoked last Friday after Mr Hawke said his presence in Australia posed a public health risk.

He argued Djokovic's presence risked aggravating anti-vaccination sentiment and causing civil unrest.

Australian Prime Minister Scott Morrison welcomed the federal court ruling, saying the decision would help "keep our borders strong and keep Australians safe".

French Open appearance in doubt

Doubts have been raised about whether Djokovic will be able to play in the next scheduled Grand Slam, the French Open, starting in late May.

The country's sports minister has said there will be no exemption from its new vaccine pass laws.

More than 95% of the top 100 male and female tennis players in their tours' respective rankings are vaccinated.