Preheat oven to 375°F (190°C).
Pat the duck dry and score the skin carefully without cutting into the meat.
Season duck all over with salt and black pepper.
Place duck breast-side up on a rack in a roasting pan.
Roast the duck for 90 minutes, draining excess fat occasionally.
Remove duck from oven and let it rest for 15 minutes.
Meanwhile, heat sugar in a saucepan over medium heat until caramelized.
Carefully add orange juice and vinegar, stirring to dissolve caramel.
Stir in orange zest and chicken stock and simmer until reduced.
Whisk in cold butter to finish the sauce.
Carve duck and serve with orange sauce drizzled on top.