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On the Karpov v. Fischer debate, I think Fischer knew he would've lost. Fischer demanded another ridiculous laundry list of conditions he needed to play, but there one of them was pivotal: a rule that in the event of a tie, the Champion (Fischer) would retain the title.
Had the Russian ever demanded such a rule, Fischer would've berated them as cheats. Now here he is, not having played a game in 3 years, demanding rules unfairly tilted in his favor. Fischer likely believed he didn't have what it took to win. Maybe not necessarily because of his opponent (Karpov), but maybe Fischer realized he had lost his nerve, as evidenced by Fischer's virtual disappearance afterward.