He needs to go into this match believing he can win it and going for his shots. If he plays defensively, Novak will dominate the rallies. Even though I'm a huge Novak fan, Grigor is my favourite player and I'm hoping for an upset.
Yes, he really needs to do that, but he also have to keep that thought no matter what the score is and keep fighting until the last point and not give up after losing the first set, if he gets broken or something like that. I think right now Grigor won't be able to challenge Nole and I expect something like 6-3 6-2 win for Nole, but I'll be really happy if Grisho makes me shove those words back into my mouth!
Good luck to him and hopefully he will show us what he is capable of. The most normal thing is that Grigor will lose tomorrow, but I think that will be a great experience for him and if I'm not mistaken I think some months ago he said in an interview that he wants to face Djokovic (that was probably when Nole was still number 1 in the world) and now he will have that chance