Well, I will give you the biggest upset that I have ever seen. Once in club chess rated at a long time control GM Darryl Johansen lost to a 1300 rated player. I don't know what Johansen's rating was at that time [it was late 80's early 90's from memory]. That is ONCE out of 1000's of rated club games.
So I will say 0% chance in the sense of it being so asymptotically unlikely.
Of course, I'm talking about games when both players want to win.
I also don't buy %0 percent chance, because I think I can get a win against a Boris Gelfand type blunder.