Also, you don't want to expose Deathrite needlessly unless you have something like a Dark Confidant after that. First turn Thoughseize allows you to sculpt your plan significantly.
Very few combo decks can go off on turn 1. The ones that do get hurt a lot by it (Belcher) still can top-deck into something quickly, and they run up to 11 kill spells, so chances are reasonable they will hit it. That being said, there's no question about turn 1 TS vs. Belcher, just saying, it might not be the one all end all.
Against other combo decks, it is significantly better to play Shaman first so you can vault ahead. You need a bit of luck to win. If they have it (and they might), c'est la vie.
The bigger concern is that they might be able to use Brainstorm to hide their best cards. If this is the issue, then try to get TS going when they cannot Brainstorm.

