Place the sugar, paprika, cumin, fennel, cayenne, garlic, grapefruit zest and segments, and salt in the bowl of a food processor. Purée until smooth and transfer to a large bowl. Add the pork tenderloin and toss to coat. Cover with plastic wrap and let sit at room temperature for 15 minutes.
Transfer the pork to a parchment-lined baking sheet fitted with a wire rack. Cook about 20-45 minutes until the internal temperature of the pork reaches 140˚F, and the outside is lightly caramelized. Let rest for 10 minutes, then slice and serve.