Pare sweet potatoes, removing any dark spots; cut crossqwise into 1-inch thick slices. Place in a medium bopwl with water to cover. Soak 10 minutes, changing water twice during soaking time. Drain.
Cut lemon crosswise into 1/8-inch thick slices. Place potatoes, 2 1/2 cups water, the sugar, and salt in a 2-quart saucepan; heat to boiling on high heat. Reduce heat to medium and cook gently 10 minutes. Add lemon slices. Continue cooking, adding more water if necessary, until potatoes are tender, about 20 minutes longer. Drain; place in serving bowl and serve hot or at room temperature.