Federer's game plan to Djokovic should stick what he did against Djokovic in past three years, keep changing pace and rhythm by drop shot, slice and normal groundstroke shot.
This years, two loses prove that toe to toe does not work to against Djokovic.
And maybe Federer should learn some knowleges of Sampras groundstroke shot, especially backhand.
1. Sampras able to swing his backhand to make big cross court angle to opponent oppiste position in the middle of the court, consistently.
2. Smapras able to swing his forhand and use opponent strength to make suddently changing course forehand.
I saw him play similar shots couple of times on French open.