Surely he didn't follow the old cookie-cutter formula of take a day off before game day, play blitz matches, or some other standard ritual?
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This is a question for people who knew him well personally. Anyway Fischer lived and breathed chess almost daily since the day in which he learnt the rules, until well into the first match vs. Spassky, so we have no reasons to believe that he changed his lifestyle before tournaments, i.e. he constantly played and studied chess, at least until his life's main mission changed and became the one to "unmask the Jewish conspiracy". But this happened after the first match vs. Spassky. However it is evident that the mature Fischer was aware of the importance of being fit physically, so we have to suppose that he did some exercise, especially before important matches and tournaments.