Fighting Defensively is something you do as part of an attack or full attack action, and the effects last until your next turn. If you haven't had a turn yet, you aren't fighting defensively. (Note also that this means the wizard generally can't fight defensively while taking, for example, the Cast a Spell action.)
Your Dodge opponent is designated 'during your action'. You can select a new opponent 'on any action'. It seems likely that this is the 'turn in the initiative order' use of the word 'action'; otherwise, you could use the free action 'Speak', which can be taken at any time (even when it is not your turn) to switch your designated Dodge opponent to whichever foe is currently acting.
If we assume this to be true, it means that until your turn, you cannot select (or change) your Dodge opponent. Also, the Dodge feat grants a Dodge bonus, which (unless the wizard has Uncanny Dodge) means that it does not apply while he is flat-footed... and he is flat-footed until his first turn in the initiative order.
-Hyp.