TOKYO (AFP) Towering Frenchman Teddy Riner, now a triple world heavyweight judo champion, faces an unlikely possible challenge from Japans Olympic winner turned mixed martial arts fighter Satoshi Ishii. Ishii, 23, quit judo after winning the over-100 kilogram title in his Olympic debut at the 2008 Beijing Games, but said he was considering competing in the sport again at the 2016 Rio de Janeiro Games as a US citizen. I hope Mr Riner will keep on working hard until then and I want to finally have it out with him, Ishii told Japanese media Thursday when Riner retained the over-100kg title on the first day of the world judo championships in Tokyo, his third consecutive world heavyweight win. Ishii and Riner were rated as top favourites in Beijing but never clashed as the Frenchman stumbled in the fourth round. The Guadeloupe-born 21-year-old fought back through consolation matches to grab a bronze in his first Olympics. In late 2008 Ishii gave up judo after the sports national association did not approve his plan to double as a judoka and a professional fighter in mixed martial arts. After months of preparations and media hype, he made his professional debut last New Years Eve and is planning to base himself in Los Angeles where he had a summer training camp. I have learned that professionals can take part in judo competitions in the United States, Ishii said, adding he might obtain US citizenship after he is granted permanent residency. I will probably compete in the Rio Olympics as a member of the US national judo team, he said. I will probably get gold. A US green-card holder is normally required to live in the United States for five years before he or she can apply for US citizenship. But Ishii, 181cm and 110kg, said the process might be eased for him due to his Olympic ambitions. Riners collection has expanded to four world championship gold medals matching three Japanese Shozo Fujii, Yasuhiro Yamashita and Naoya Ogawa and David Douillet, one of the best French judokas of all time. Riner, 204cm (six foot eight) and 129kg (284 pounds), will attempt to be the first man to win five world titles when he competes in the open category in Tokyo on Monday.