But don’t you know, there are some things that can beat smartness and foresight? Awkwardness and stupidity can. The best swordsman in the world doesn’t need to fear the second best swordsman in the world; no, the person for him to be afraid of is some ignorant antagonist who has never had a sword in his hand before; he doesn’t do the thing he ought to do, and so the expert isn’t prepared for him; he does the thing he ought not to do: and often it catches the expert out and ends him on the spot.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

A Connecticut Yankee in King Arthur’s Court, ch. 34 (1889)
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Origin of more simplified versions of the phrase. More discussion: The Best Swordsman in the World Doesn’t Need To Fear the Second Best Swordsman – Quote Investigator.