Two U.S. billionaires are betting on rival cities, Tokyo and Osaka, to be the first in Japan to open casino resorts — once the government gives the go-ahead to legalize gambling.

Japan is one of the world's last untapped gaming markets and could become the third-largest gambling destination after Macau and the United States, with annual revenue of over $40 billion, according to broker CLSA.

Lawmakers who support legalizing casino gambling hope to see initial draft legislation this year, with the first resort opening by 2020, when Tokyo hosts the Olympic Games.