Place chicken breast between 2 pieces of waxed paper and pound with a meat mallet until they are an even thickness.
Whisk lemon juice, zest, olive oil, garlic, thyme, salt and pepper in a small bowl. Place chicken breasts in a plastic ziptop bag and add marinade. Place in a bowl to prevent leakage and place in fridge overnight.
When ready to cook preheat grill to medium high. Make sure grates are clean and wipe with an olive oil spritzed paper towel.
When heated place chicken breasts on grill and do not move. Cook 8 minutes on one side then flip. You may have to turn grill down to medium depending on the power of your grill.
Begin checking the internal temperature with a meat thermometer. When temp reaches 165 degrees, remove chicken and place on a plate covered with foil.
Slice and serve immediately with any pan juices. Garnish with more thyme.