British Grandmaster Nigel Short has told The Sunday Telegraph he believes he has faced the American former world champion in almost 50 internet games in the past year.
Fischer retreated from public view after taking the world title in possibly the most famous match of all time, a Cold War battle against Russian world champion Boris Spassky in 1972.
Since then his whereabouts have remained a mystery and he has surfaced only once - in 1992 - for a 20th anniversary rematch in Yugoslavia with a $5m prize, which he won.
 From this deliberately unpromising position emerged moves of extraordinary power  |
Nigel Short Chess Grandmaster |
"I am 99 percent sure that I have been playing against the chess legend," he said. "It's tremendously exciting."
Short said he was initially sceptical when told by a Greek Grandmaster last year that Fischer had been playing speed chess anonymously on a website, the Internet Chess Club.
In speed chess, known as "blitz" in the chess world, each player has a three-minute time limit per game.
Despite his misgivings, Short eventually arranged to play the unknown opponent, and in October last year lost the first of their four confrontations 8-0.
'Absurd' moves
Short said his adversary's style of play was intriguing.
"My unseen opponent began with some highly irregular, if not totally absurd, opening moves - shifting all his pawns forward by one square. These were moves that no Grandmaster would ever play."
Short said he immediately suspected a hoax, but became aware there was method in the apparent madness.
 Nigel Short believes Fischer is better at speed chess than Kasparov |
Short is one of the world's best speed chess players, and drew a speed chess series 6-6 with then world champion Garry Kasparov in 1995.
But he told the Telegraph: "In my opinion Fischer is a much stronger speed chess player than Kasparov, which is incredible when one considers that at 58 he is virtually a geriatric in terms of the modern game."
'Undiscovered symphony'
During the internet games, Short chatted online with his mystery opponent, whom he said showed great knowledge of the major chess players of the 60s - Fischer's most active period.
The most decisive "proof" came when Short asked his opponent if he knew of Armando Acevedo, an obscure Mexican player.
The immediate reply was: "Siegen 1970." Fischer had played Acevedo at the Siegen Chess Olympiad in 1970.
"The guy was obviously trying to tell me something," Short told the paper.
The British player fears his revelations may mean that Fischer - if it is him - will be unwilling to play Short again on the web.
But he said the games would have a lasting effect on him.
"To me, they are what an undiscovered Mozart symphony would be to a music lover," he said.
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