You can get Kindle editions The Fredric Brown Megapack (2 Book Series) for $2, which includes 60 of his stories, many of which have great surprise endings. I bought it and I see many of my favorites here, including "Arena."
"Arena" is a science fiction short story by Fredric Brown that was first published in the June 1944 issue of Astounding magazine. Members of the Science Fiction Writers of America selected it as one of the best science fiction stories published before the advent of the Nebula Awards, and as such it was included in The Science Fiction Hall of Fame Volume One, 1929-1964.
The Star Trek episode "Arena" had some similarity to this story, so to avoid legal problems, it was agreed that Brown would receive payment and a story credit. An Outer Limits episode, "Fun and Games", also has a similar plot, as does an episode of Blake's 7, titled "Duel".
If you are not yet familiar with Fredric Brown, I'm envious of the treat that's in store for you.