I love watching Diaz fight, he is one of my favorite fighters... but he gets absolutely slaughtered by GSP (loses a UD
GSP certainly wouldnt be toying with Nick standing, he wouldnt take that risk... but his Jab and leg kicks would clearly be there... He has some reach on Diaz and would use it to keep him on the outside... And his double would be there clearly as well, Diaz has questionable TDD and GSP has the best takedowns in the game... Nick has a sick ground game, but GSP has fought guys like BJ and Condit's guard at length and pushed through, doing some damage himself.
Also of note. Diaz is a volume puncher... Carlos packs some serious power, while Diaz likes to pick apart and break down his opponents... If the fight stayed standing the entire time, Nick can do that to anyone at WW, GSP included... but the second GSP feels threatened standing, Nick is on his back.
Nick would have to really tag GSP and have him really dazed for GSP not to be able to get him down.
Not a bad fight, based on Diaz's accomplishments and his place in the WW division... but Id put Hendricks above him at the moment.