I think I know what you're getting at so I'll give it a go.
The D&D 3.5 texts mention mage duels being set up in a pistols at dawn / fencing kind of fashion. This was, as the books described, a very formal affair. It was very tedious like you say because of 'rounds' and formal arrangements. The thing I learned from all this though is that mages can duel and fight in a one-on-one fashion even in regular d20 combat.
The specific details do depend on your system, but I suggest including things that need Concentration and Spellcraft checks and the like. It allows for strategy, and it means you have more than just the linear Magic-vs-Magic - unless of course a Harry Potter-esque style is what you're after!
Magic will be naturally more fast paced then melee combat in a D20 system as you don't need to worry about being in melee range or moving through threatened squares, but you always could mix it up with touch attacks. This is very strategy based, but strategic to the player and not the rules so much. Try easing the players into the setting that allows the exploration of magical combat and duels maybe.