Mostly agreed with Forbin, except I've been toying with the idea about how to use warp and impulse engines at this speed, in addition to shields and deflectors. I doubt the engagement would really be as simple as "hit him before he hits you." Your Klingon attacker might take several hits and chose to disengage, but if he's really pissed off you're going to still be fighting him for another ten minutes before one of you can claim victory.
I figure out it's like swords and arrows. (Bear with me on this one) at long range, you can shoot torpedoes at the other guy (arrows) or close to short range and hammer him with phasers (swords). Sword duels are only quick and brutal when they don't involve armor or some protection; if you have a short sword in one hand and a shield in the other, it might take you a minute to get past that shield and get some good happy stabbing done. So, exactly what tactic you use depends on what the other guy is equipped with. Someone with, say, deflector screens tied into his warp drive is going to be able to repel your torpedoes almost indefinitely, so you're going to have to get in close with phasers and either knock out his deflectors or force him to transfer that power to his engines just to get away from you. At that point, you either finish him off with the phasers, or you let him get some distance and hit him with torpedoes (or both). If he's got a lot of heavy armor, you don't have a choice: a phaser or torpedo strike will have to hit him in a sensitive spot to do any damage.
Starship combat would probably degenerate into knife fight ranges more often than you'd expect, as ships get closer and closer in an attempt to thwart each other's defenses and make more of their own shots hit. Jumping back out to a distance -- if the other guy gives you time -- can give you a chance to catch your breath and regroup before pressing the attack again, but the nature of heavy shielding and deflectors means it's still more likely to be a sword fight than a sniper duel.