I believe I read this story in a fantasy anthology in the early nineties. It was about a child who is the only other human being in a tiny world created by a wizard who is all-powerful within their reality. He has created and destroyed a number of different things to amuse himself, but without much success. It seems somehow that the child is different, and that he has some powers of his own. At the end of the story, the wizard decides to uncreate the child, only to realize that the child actually created him instead of the other way around.
The overall concept is quite similar to the Borges story referenced in this question, but it is not the same story. This one is considerably longer, more recent, and presumably first published in English.