I always teach running away as an option for an ending to tenkan undo. Particularily with a knife.
My jo teacher's aikijujutsu teacher,(a very very big name that I don't feel like misquoting) used to say he could potentially take a knife. This is a top dog here, not just some schmutz. In aikido we practice sword taking spear taking, and anything else that we might like.
If one of the top dogs of a combative martial art was humble enough to admit that he MIGHT be able to take a knife I think the rest of us might reconsider how far our techniques apply to sincere combative combinations.
Irimi, Tenkan, Atemi, Distraction, then when you have a significant advantage on a deadly opponent, USE IT.
RUN LIKE HELL.
(and run faster!)