Preheat oven to 325 degrees F (165 degrees C).
Bring water to a boil in a large pot.
Cook sweet potatoes in boiling water until tender, about 7 minutes; drain.
Transfer potatoes to a large mixing bowl; mash with a potato masher until smooth.
Add 3/4 cup brown sugar, Tennessee whiskey, 1/4 cup softened butter, cinnamon, and nutmeg to the potatoes; stir to incorporate.
Spread mixture into a 1-quart casserole dish.
Melt 1/4 cup butter in a small skillet over low heat.
Stir 3/4 cup brown sugar into the melted butter until incorporated.
Add pecans to skillet; cook and stir until the pecans are completely coated in the butter mixture, 2 to 3 minutes.
Top sweet potato mixture with the pecans.
Bake in preheated oven until browned around the edges, about 25 minutes.