In a large mixing bowl, combine olive oil, vinegar, soy sauce, Worcestershire, lemon juice, sugar, rosemary, mustard, salt, pepper, and garlic powder. Whisk until smooth.
Reserve 1/2 cup of the marinade in a sealed container for basting or serving later.
Place chicken in a large resealable bag or shallow dish. Pour in the rest of the marinade. Seal or cover and evenly coat the chicken by massaging or pressing gently.
For faster absorption, poke the chicken with a fork a few times. Marinate in the fridge for at least 30 minutes, ideally 4–6 hours, up to 24 hours.
Preheat grill to medium-high. Oil the grates. Remove chicken and let excess marinade drip off. Grill each side for 5–6 minutes or until internal temperature reaches 165°F (74°C).
Brush reserved marinade over chicken once or twice while cooking. This keeps it moist and boosts flavor.
Transfer cooked chicken to a plate. Cover loosely with foil and rest for 5 minutes before serving.
Optional: To turn reserved marinade into sauce, bring it to a boil in a saucepan and simmer for a few minutes until thickened.