the top 100 players in the world is a small list, and at this level you don't need to worry about anyone below that (until they get higher) as you are enough better than them that you can beat them. Thus for tournaments you don't have worry as you have already studied everyone you need to worry about before the tournament. Odds are you know ahead of time who is in the tournament and so you have a couple weeks to study all possible opponents.

Remember for the best players in the world chess is their full time job. They spend 12 hours a day on the game. They are earning enough from tournaments to support life.