I generally use a base combo to fight, with obvious in the moment variations as I see fit. The way to start in a 1v1 is to Q in, get in an AA to proc your W, then E, and repeat minus the Q. If they flash or have superior movement speed/escapes, you can R them. I only recommend this if you either have vision, or really need a kill to get on track. This is essentially the same way to play Teamfights, only here you have to prioritize. I always go for the enemy that does the most damage first. Since they are often squishy, mid game it is very possible to 100-0 them in a matter of seconds. I skip using R to catch people in Teamfights until the fight begins to break up. You can smite to keep up with a straggler and then Q them to end them instead of popping the Ult. R can then be used to pick off any remaining stragglers.