Place an oven rack in the center of the oven. Preheat the oven to 325 degrees F.
For the rub: In small bowl, combine the sugar, chili powder, smoked paprika, salt and pepper.
Pour the beef broth into a 9- by 13-inch baking dish. Using a paring knife, score the surface of the meat all over. Massage the rub into the meat. Cover the dish tightly with foil and roast until very tender, 3 1/2 to 4 hours, adding water 1/2 cup at a time if the roasting juices start to dry up.
For the sauce: In a food processor, combine the garlic, celery, onion, tomato sauce, vinegar, maple syrup, mustard, smoked paprika, chili powder and salt. Blend for 1 to 2 minutes until smooth, scraping the side of the food processor bowl as needed with a spatula. Transfer the sauce to a medium saucepan. Bring the mixture to a simmer over medium heat. Cook, stirring occasionally, until thick, about 30 minutes.
Brush 1/3 cup of the sauce over the surface of the brisket.
Slice the brisket into 3/4-inch pieces and arrange on a serving platter. Serve the extra sauce alongside.