Diaz will tire. GSP's cardio is better (for MMA) and carrying GSP's weight around getting pummeled will have him gassing by the 5th. I'd bet on Diaz is taking this fight lightly as well.
GSP's sub defence is more than good enough to avoid Nick on the ground and rain down elbows and punches at will. Nick is getting smothered and beat up. No other outcome. On the feet all GSP has to do is jab, and take him down whenever Diaz moves forward. No way in hell Diaz will back him into the cage and get off punches.
the [art you are forgetting is nick never gets tired in any situation, if you have seen his fights with diego and sherk which ad lots of grappling in it you would see how fresh and how active is the whole time. And as for the ground no way GSP survives long on top GSP was swept a few times by condit who is significantly worse at bjj than nick and that;s not a knock on condit but diaz is fantastic off his back. Also I have seen GSP fail on the ground with hardy too and alves so nick will be fine.