The assumptions here are that your opponent will block whenever you could win on the next move, and also that given the opportunity to force a win (by creating two different directions with two in a row) your opponent will take it, but that otherwise, your opponent's moves are random.
In that setting, your best strategy if you go first appears to be (1st move) take the center, (2nd move) move opposite your opponent, (3rd move) create two directions of two in a row (which will always be possibly unless your opponents first two moves were corners, in which case the game will tie - this is even true if your 3rd move is actually blocking - blocking will create a forced win for you).
What is your best strategy if you go second instead? Is there an opening move your opponent can make which would allow you, as second player, to force a win, instead of just a tie?