Snap off tough ends of asparagus, and cook asparagus in boiling water to cover 3 minutes or until asparagus is crisp-tender; drain.
Plunge asparagus into ice water to stop the cooking process; drain. Arrange asparagus in a 13- x 9-inch baking dish.
Whisk together olive oil, sugar, balsamic vinegar, garlic, and red pepper flakes until well blended; pour over asparagus. Cover and chill 8 hours. Drain before serving.