"The last man on Earth sat alone in a room. There was a knock on the door..."
This two-sentence horror tale is presented as a story within a story, right at the start of Fredric Brown's 'Knock', published in the December 1948 edition of Thrilling Wonder Stories.
Brown decribes how the horror in this story is all implied... what could be at the door? "But," he says, "it wasn't horrible, really."
He then tells the story of Walter Phelan, the last man on Earth after an alien race called the Zan invade and kill everyone apart from Walter Phelan and the last woman alive, Grace Evans.
Perhaps as a response to Brown's tale, in July 1957, Ron Smith produced what he called 'A Horror Story Shorter by One Letter Than the Shortest Horror Story Ever Written':
"The last man on Earth sat alone in a room. There was a lock on the door..."
Brown's version of the mini-story was supposedly subsequently submitted anonymously to a short story competition on NPR, and won that competition. Plenty of web sources cite this as the original source of the story. None of the sources have any more details about the competition, but Brown's version was published some twenty years before NPR started, so this is clearly false.
Even so, Brown did not come up with this story himself. It has existed in various forms for over a hundred years. Thomas Bailey Aldrich presented his version circa 1870:
"A woman is sitting alone in a house. She knows she
is alone in the whole world; every other living thing is
dead. The doorbell rings."
Diverging slightly from the theme, but still essentially the same story, is this, which my source tells me is a 'well known British ghost story':
"He sat alone in the dark, afraid. Someone
put matches in his hand."
This is actually the shortest version I've come across. I think I like Ron Smith's version ("there was a lock on the door") the best though. Not only is there something else out there, the last man wants to keep it out there...
smartalix says "...or someone/something else wants to keep him in..."
Befey says "Or, he's locked in the room with no way out... starving to death is pretty horrific."
Neither of those even occurred to me. I bet a psychoanalyst would have fun with this.
Footprints says "a lock of hair from the 2nd to last person on Earth, the one he dismembered and hung his intestines on the wall as a trophy you mean. Muahahahahahahaha!!!!!"
I'm glad that one didn't occur to me.
allseeingeye says "last dude hears knock."
Thanks to Erik Arthur, Ted Ball, Sarah Prince, and Richard Harter for helping me research this.