1. Season the thighs with pepper and garlic powder on both sides. Put the thighs in a 1-gallon zip-lock bag.
2. In a small bowl, stir together the Worcestershire and teriyaki sauces until evenly combined, then pour into the bag with the chicken. Seal the bag and place it in a pan in case it leaks. Refrigerate for a minimum of 2 hours, or overnight for best results.
3. Set a grill to 450°F or medium-high heat.
4. Grill the chicken thighs for 5 to 6 minutes on each side, or until the internal temperature reaches 180°F. Remove from the heat and enjoy!
