Hooper and Whyld call Fischer's achievement "the most remarkable achievement of this kind", noting that the 1963/64 U.S. Championship was "a tournament of about category 10."
(Hooper & Whyld 1992, p. 81) Hooper, David; Whyld, Kenneth (1992), The Oxford Companion to Chess, London, New York: Oxford University Press, ISBN0-19-866164-9