I mean, there's been one other Trek episode that was an adaptation of an earlier, non-Trek science fiction work: TNG's "Tin Man"...
What about Fredric Brown's "Arena"?
According to Inside Star Trek,
by way of Memory Alpha [http://en.memory-alpha.org/wiki/Arena_(episode)#Script
was not initially written as an adaptation, but rather it was one of those cases where two people came up with a similar idea independently. However, the executives decided that they needed to purchase the story from Brown anyway in order to film the episode.
I guess the hair being split here is that the episode was not conceived of
as an adaptation. I'd thought of Arena
too, and already looked it up.