Preheat the oven to 350°F (175°C).
Roll the pie crust into 10-inch circle on floured surface. Transfer it to 9-inch pie pan, fold the edges and pierce holes with a fork. First, blind-bake the crust to prevent the crust from puffing up. In order to do that, cover the crust with a foil and pour raw rice or dry beans on top. Bake for 15 minutes. Then remove the foil with rice/beans and bake for another 15-20 minutes, or until golden brown.
Once the pie crust is ready, whisk together sugar and cornstarch in a small bowl. In a large mixing bowl with whisk attachment beat the egg whites until foamy. Then slowly add sugar mixture one tablespoon at a time and continue to beat on medium high speed until hard peaks form. Set aside.
In a medium saucepan, whisk together ½ cup of water, grapefruit juice, sugar, cornstarch, flour, grapefruit zest and salt. Bring the mixture to a boil over medium heat, stirring constantly. To temper egg yolks, add about half cup of grapefruit juice mixture into the egg yolks and stir vigorously. Return the tempered egg mixture back to the saucepan. Continue cooking for 3-5 minutes, continuously stirring. Throw in the butter. While butter is melting in the filling, briefly whisk the egg whites for a minute. Transfer the hot filling into pre-baked pie crust and immediately spread the meringue over the hot filling making sure to seal the edges. Bake the pie for 8-10 minutes, or until meringue is golden brown. Cool completely for at least 2-3 hours before serving.