Working over a bowl to capture the juice, remove rind of grapefruit with a sharp knife, and cut into sections. Squeeze to extract ½ cup juice, and set aside.
In a small bowl, combine fennel, black pepper, salt and garlic. Use mixture as a rub for chicken, then sprinkle chicken with flour.
Heat oil in a large nonstick skillet over medium high heat. Saute chicken until browned (approximately 4 minutes per side).
Transfer chicken to a platter.
In skillet, add reserved grapefruit juice, sugar and red pepper flakes, scraping up the browned bits. Cook for one minute, then return chicken to skillet, cover and simmer until done (approximately 12 minutes).
Stir in grapefruit sections and olives and simmer one minute.