Oh, I love that sort of stuff. My current main project is a blend of the two, perhaps leaning a bit more towards fantasy. I don't have dragons (they're extinct, the setting's equivalent of dinosaurs) but there are spaceships (only one actual high-tech vessel, but many magical oars-and-sails type ships that traverse the starry void) plus various kinds of gods, angels, demons, devils, Lovecraft-style abominations, the Midgard Serpent, orcs, dwarfs, elves, artificial intelligence, robots, magic, magical and mundane weaponry ranging from sharpened sticks to plasma rifles and laser cannons... It all somehow balances out, I think. I hope. At its heart it's supposed to be a silly and quite surreal comedy, anyway. Whenever it isn't trying to be a poignant tragedy or a gripping war story instead. It's a bit all over the place in more ways than one. More than two, even.
I've mixed the two genres in other works as well, I could be here all evening listing examples.
I can't remember an awful lot of examples I've read. There are a couple of "fantasy-world-is-actually-post-apocalyptic-earth-or-something" ones I can think of off the top of my head, and I'm gonna spoiler-wrap because generally it's built in as a twist or revelation.
Mark Lawrence's The Broken Empire and The Red Queen's War are both this, taking place in the same version of post-apocalyptic Europe (mainly Europe, anyway). Been ages since I read them, aaaaand I actually remember now I never finished RQW. They're fun, anyway, if you like this sort of thing. It's kinda-sorta subtle until it isn't, and there are some very tasty Easter eggs along the way. I had some jolly good laughs that time some random far-future mercenary busted out a guitar and started singing a warped version of, um, was it American Pie, perhaps? As I said, it's been a while.
Joe Abercrombie's Shattered Sea trilogy is also this, though less overtly from what I recall.
Gene Wolfe's Book of the New Sun also falls into this category, somewhat, somehow. It presents as fantasy, when actually it's... weird. I guess you'd call it sci-fi, but I feel like it just outright transcends genre. It's its own unique beast. Highly recommend it. I did once upon a time work on such a project myself, but it's been stuck in development hell for absolute æons.
I'm sure there are tons of books that more directly combine fantasy and sci-fi, but all I can think of at the moment is stuff that places itself more or less in the steampunk bit of the spectrum. I'd love to read some, if I could find them. I think it's great.