Both usually go, somewhat amusingly, hand in hand, but I think they should be linked to the "mastery" discussed in the topic. Fear of both failure and success are, in short, fear of change, fear of losing control, fear of not being a master anymore.
It is indeed quite common to trade a, sometimes illusory, feeling of easiness with the known current situation for the potential uneasiness of an unknown situation given as one the possible outcomes of the failure/success. Particularly in the last case, I have seen that it is not unusual to rationalize that we have already gained achieved should not be "gambled" against the changes that would result from the success of the activity we are procrastinating.
In short, the "I'll do it tomorrow" is usually a "I'll do it when I will feel confortable with outcomes I now consider most likely". I personally have discovered that focusing in the positive aspect of the outcomes I consider most likely, even in case of failure, is usually my best weapon against procrastination.