Chelsea football club owner and superyacht enthusiast Roman Abramovich slipped out of the top 10 richest Russians in Forbes magazine's 2013 ranking published on Thursday.

The flamboyant steel, mining, real estate and telecommunications tycoon has lost $1.9 billion of his fortune and is now worth just $10.2 billion, enough for 13th place, the Forbes list said.

Forbes did not explain Abramovich's fall. He is noted for having one of the world's largest yachts that he uses to travel the world, and which recently docked in New York for the birth of his son.

But it said that he has a $1 billion art collection, and that he bought 40 works by top conceptualist artist Ilya Kabakov for $60 million earlier this year.

The list is headed for the second year running by Alisher Usmanov, an Uzbek-born Russian tycoon whose $17.6-billion fortune comes from original investments in mining and timber, which have since expanded into media and other interests.

Usmanov was ranked as the 34th-richest person in the world by Forbes, while Russia's number two Mikhail Fridman, a former co-owner of the TNK-BP oil company, was ranked 41st worldwide with $16.5 billion in wealth.

The top five was rounded out by the $15.4 billion of Leonid Mikhelson, a major stakeholder in the Novatek natural gas company (ranked 47th in the world); the $15.1 billion of Viktor Vekselberg, another former TNK-BP co-owner (ranked 52nd in the world); and the $14.8 billion of Vagit Alekperov, president of Russia's number two oil producer Lukoil (ranked 55th in the world).