Flow is being completely involved in an activity for its own sake. The ego falls away. Time flies. Every action, movement, and thought follows inevitably from the previous one, like playing jazz. Your whole being is involved, and you're using your skills to the utmost.
Fearless is not the absence of fear. It's not being completely unafraid. Fearless is having fears. Fearless is having doubts. Lots of them. Fearless is living in spite of those things, every day.