Well, it's the fight game. Anyone can lose, everyone does at some point. (Well, almost anyone). It's all so random. If you think you can predict everything you are in for a rude awakening.
That being said I think GSP is one level up from Diego. I like Diego a lot (his fighting style) but what can ge do against GSP. I think GSP's striking is better, and I don't think Diego could take him down once in 10 tries. GSP is amazingly strong and he trains with top-flight wrestlers who are bigger than him. And I think that even if he did get GSP down, GSP could still handle him.