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