Near Changi Airport, at 32, Changi North Crescent, stands a squat building. Ringed by a concrete wall, Le Freeport is patrolled by armed guards and monitored by sensors.

Long corridors lead to strong rooms and vaults, where the temperature and humidity are carefully maintained at 20 deg C and 55 per cent respectively. After all, they contain veritable treasures: works of art, gold bars, antiques and vintage cars.

It is also here that one of the world's most expensive paintings - No. 6 (Violet, Green And Red) by abstract expressionist Mark Rothko - is being kept for the last three years.

It was acquired for US$80 million (S$111.8 million) by one of Le Freeport's co-founders, Swiss businessman and Singapore permanent resident Yves Bouvier, who also uses corporate vehicles to buy and sell valuable works of art.

It was sold, in turn, to Russian fertiliser magnate Dmitry Rybolovlev for €140 million (S$213.2 million) in 2014. Mr Bouvier would have pocketed a cool $100 million.

But it was not to be.

On Feb 25, 2015, Mr Bouvier made his way to Mr Rybolovlev's penthouse in Monaco, expecting to discuss the outstanding payment. What he did not know was that the Russian oligarch - via two companies he used to buy art acquired by Mr Bouvier - had filed a criminal complaint, accusing the Swiss of fraud. Mr Rybolovlev complained that Mr Bouvier inflated the prices of the 38 pieces he bought for him.

When Mr Bouvier arrived, he was arrested; he was detained for three days before being released on bail.

Since then, Singapore has become their battlefield in a clash that has gripped the international art world and cast a spotlight on the unregulated art market.

The two companies controlled by Mr Rybolovlev filed a lawsuit here against Mr Bouvier and his corporate vehicle, seeking to recover US$1 billion in alleged mark-ups.

The main dispute is over the nature of their relationship - whether Mr Bouvier was an agent of the companies and therefore owed them fiduciary duties, or an independent seller who was entitled to mark-ups.

Both camps hired Singapore's top legal guns. Mr Rybolovlev was represented by senior counsel Alvin Yeo and, later, senior counsel Davinder Singh. Mr Bouvier was represented by senior counsel Edwin Tong.

Mr Bouvier argued that their dispute should be heard in Switzerland as their dealings took place in Geneva, witnesses are in Europe and documents are in French.

But Mr Rybolovlev argued that they would be disadvantaged in Switzerland, which does not recognise legal concepts they can pursue under Singapore law.

Earlier this month, Mr Bouvier scored a small but significant win. The Court of Appeal agreed the dispute should be heard in Switzerland.

Mr Bouvier told The Sunday Times he was "delighted".

"Rybolovlev's attempt to take the case around the world to better serve his own agenda has failed. I am very confident that I will be vindicated, so I can work towards putting this matter behind me, which has been a massive disruption to my personal and professional life."

Mr Rybolovlev's adviser, Mr Sergey Chernitsyn, said the decision "merely constitutes another procedural incident in this complex international litigation".

In an interview with The New York Times in 2015 about the dispute, Mr Rybolovlev said: "If the market were more transparent, these things wouldn't happen."

BILLIONAIRE SAYS HE WAS DUPED

The saga began in 2003 when the two men met in Geneva.

Mr Rybolovlev was a cardiologist who made his fortune in fertilisers. In 2011, he moved to Monaco; he is the majority owner of Monaco football club. He made headlines in 2014 for a multibillion-dollar divorce judgment and is known for lavish property purchases, including Mr Donald Trump's oceanfront mansion in Florida and a Greek island.

Mr Bouvier is an art transporter whose family business specialises in shipping, packing and storing works of art. He exported the Geneva concept of a freeport - a tax-free storage facility for art pieces and other prized possessions - to Singapore and Luxembourg. In 2009, he moved here.

For more than a decade, Mr Bouvier played a key role in building Mr Rybolovlev's US$2 billion art collection. In a climate where secrecy is key, Mr Bouvier's extensive network of contacts enabled him to locate and acquire art pieces. After each piece was delivered, Mr Rybolovlev paid Mr Bouvier 2 per cent of the value of the work.

Their relationship unravelled on New Year's Eve in 2014, when art adviser Sanford Heller told Mr Rybolovlev a Modigliani nude, Nu Couche Au Coussin Bleu (Reclining Nude With Blue Cushion), was sold for US$93.5 million. Unknown to Mr Heller, Mr Rybolovlev was the mystery buyer. But he paid US$118 million for it - meaning Mr Bouvier marked the price up by more than US$24 million.

Separately, he learnt from a New York Times article that another painting in his collection, Leonardo da Vinci's Le Christ Comme Salvator Mundi (Christ As Salvator Mundi), was sold for between US$75 million and US$80 million. But he paid US$127.5m for it.

Mr Bouvier has a different take. He said their relationship soured because Mr Rybolovlev did not take kindly to being pressed to make full payment for the Rothko piece.

WHAT NEXT?

The matter does not end here. Now that the Singapore route to recover his money has been closed off, Mr Rybolovlev has to decide if he wants to take the Swiss path.

Mr Chernitsyn said the plaintiffs could still return to Singapore and ask for the stay to be lifted if the Swiss courts decide they do not have the jurisdiction. But Mr Bouvier's Paris-based lawyer Ron Soffer said: "If the plaintiffs choose to file a lawsuit against Mr Bouvier in Switzerland, it is impossible for me to imagine how the Swiss courts can refuse to hear the case."

The one thing tying the dispute to Singapore is the Rothko, currently under the court's safekeeping. A commercial litigation lawyer not involved in the case said both sides can apply to the court to release it, but this will raise the contentious issue of who owns the painting.

If no suit is filed in Switzerland, Mr Rybolovlev has no more civil claims against Mr Bouvier, said the lawyer, who declined to be named. And if a suit is filed in Switzerland, Mr Bouvier can ask the Singapore courts to let the Swiss courts deal with custody of the painting. Either way, the Rothko is not likely to be moved from Changi without another fight.