I appreciate your confidence. And I'm sure you're a bright and diligent student. But you've likely never been in an academic environment where you're surrounded by people who are as equally intelligent and determined as you. In fact, some of these people are intellectual mutants---scary smart.
Granted finishing in the upper third of your class sounds quite easy. You, like most of your fellow students, have probably achieved a great deal of academic success in the past so top third sounds like a cakewalk. But from a purely statistical standpoint, you're almost twice as likely to end up in the bottom 65% than in the upper 35%.
Anyway, I wish you all the success in the world. Good luck!