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.