JRR Tolkien and CS Lewis were colleagues and Lewis referred to Numenor and the True West in his science fiction trilogy before Tolkien published his fantasy trilogy.
Tolkien and Poul Anderson independently wrote fantasies based on Norse mythology.
Anderson also wrote sf, including alternative histories.
SM Stirling specializes in alternative histories with some Andersonian allusions, including a series where some Change survivors base themselves on Numenoreans. For them, Tolkien's Middle Earth History might as well be real history.