Old Scratch. Scrat, the house-demon of the North. (Icelandic, scratti, an imp.) (See Deuce, Nick , etc.)
(A). One who in a race starts from the scratch, other runners in the same race being a yard or so in advance. The scratch runner generally is one who has already won a similar race.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894