Had Harrison for coupla classes in early 1970s; 'round the time he was writing Rollerball & another angry tome called The Warrior (which he asked us to read in galleys for class).
For the genuine horror twist ending, tho, there's the equally dystopian "The Make-Up Man," from his Buddha in Malibu collection: when an…

OK; this is 3 yr late.
Harrison, who came into our form & theory of fiction class one day to ask for suggestions about this story he was writing—futuristic sports—, didn't care much for the original: "[The director] did everything with my script but use it," Harrison is quoted at IMDb.