Personally, I think Viriconium is the best far-future scifi I have ever read. It is also by far the most beautiful prose I have ever read in a sci-fi novel. In fact, it is so "literary" at times that it's almost not sci-fi. And yet, it's set in the far future and there are robots and a galactic civilization, et cetera. So what inspired its far-future setting? Was the author, M. John Harrison, actually reading scifi? Or did it just emerge seamlessly from his brain?
One of the influences was the desire by the author to reject the idea of a fictional world as a place. His essay, "What It Might Be Like To Live In Viriconium," addresses the topic directly, and this interview from 2002 discusses the matter further.
The early Viriconium pieces were anti-fantasy, written deliberately to refuse closure to fantasy readers and to get up the noses of fantasy writers, editors and reviewers.
Jack Vance's Dying Earth novels and stories are an influence, marked as they are by dense prose, wide vocabularies, and themes of entropy, decadence, and melancholy.
Michael Moorcock and J.G. Ballard are also influences, Moorcock in his revision of sword and sorcery and Ballard in his cold, unsentimental, and virtuosic prose style.