Kurt Vonnegut's Slaughterhouse Five is generally classified as science fiction and contains some science-fictional elements:
The main character, Billy Pilgrim, becomes "unstuck in time" and experiences large parts of his life out of order. He's also captured by aliens at one point.
It has been years since I read the book. But my interpretation at the time was that the story is constructed in the same way as for example K-Pax. The sci-fi elements are introduced during a horrific period in Pilgrim's life, and it's somewhat ambiguous whether the sci-fi elements are really happening, or whether Pilgrim has had some kind of psychotic break.
I think I read once that Vonnegut didn't really like his work being labelled as science fiction. That could reflect an attitude that sci-fi wasn't considered literary enough, or maybe Vonnegut actually considered Slaughterhouse to be a story about a man pushed beyond his limits.
Is it established within the book that the sci-fi elements really do take place?