Preheat oven to 170C. Wrap the outside bottom of a 25cm springform pan with foil to prevent water seeping in while baking.
Cream together ricotta cheese, cream cheese and sugar until smooth. Mix in the eggs one at a time. Add flour one tablespoon at a time, mixing well. Stir in the sour cream and vanilla. Pour mixture into springform pan.
Place cheesecake in a large rectangular pan filled with 5cm of hot water. Carefully put into preheated oven without spilling water. Bake for 90 minutes.
After 90 minutes turn oven off, but leave the cake in the oven for another 60 minutes (do not open door).
Cool on counter for 60 minutes and then refrigerate. Serve when thoroughly chilled.