Pound the thick ends of the chicken until each breast is an even 3/4 inch and season with the salt and pepper.
Whisk the maple syrup, mustard, vinegar, Worcestershire and garlic powder in a small saucepan.
Boil the glaze over medium heat until slightly syrupy and reduced to about 1/3 cup, 3 to 4 minutes, then divide it between two bowls.
Heat a grill to medium-high, clean the grates and rub them with the oil.
Grill the chicken with the lid closed until marked underneath, 5 to 6 minutes, then turn it.
Cook the second side for 3 minutes, brush with glaze from one bowl, turn and brush again; repeat once until the thickest part reaches 165°F.
Rest the chicken for 5 minutes and serve with the untouched bowl of glaze.