I personally love playing against Lee Sin as Fiora.
You just need to take from your build a bit to keep on par with him.
I usually grab Wriggles first when Top vs Lee Sin, and that's all the sustain you really need. If he's outfarming you, you can't go wrong with a Philosophers stone aswell.
I level my Riposte first for that bonus damage and the nice shield for when he try's to Kick to you, or simply for poking him. Steroid gets leveled second obviously, with one point of lunge at level 3 for chasing.
You have to play aggressive with him to make the best use of your passive, Steroid, Lunge in, Riposte, and whack at him a bit, then Lunge out if you have to, or continue the chase. You Ultimate allows you to tower dive too. Early game, you do more damage than him and attack just as quick with your steroid up. Use this to your advantage, and just harass the hell out of him. Use Wriggles to ward obviously so you can afford to push and bully him.