1. Assume the guy with the knife isn't proficient in martial arts (why else does he have a knife?).
2. Assume the guy isn't trained in the proper use of the knife (otherwise, you are just screwed anyway).
3. Find a makeshift parrying/entangling device, like a jacket, for quick protection/distraction, go for his knee, and get out as fast as you can. His focus will be on the knife, he thinks your focus will be on the knife. Hopefully you will only get one or two slashes.
4. ???
5. Profit!