Preheat oven to 325 degrees F and place crust into a 9-½" pie plate. Gently press the bottom and sides down and flute edge with your fingers. Refrigerate until ready to use.
Beat sugar and butter with an electric mixer. Add egg yolks and beat well. Beat in milk and flour. Fold in lemon juice and zest.
In a separate bowl, beat egg whites until stiff. Fold into lemon mixture and mix well by hand. Pour batter into crust and bake for 50-60 minutes or until a toothpick inserted in the center comes out clean. Let stand to cool completely and cover and refrigerate. When ready to serve, remove from fridge, slice and top with whipped topping.