Rinse the chicken breasts and pat dry. In large, shallow nonreactive dish, whisk together the olive oil, garlic, grated lemon zest and juice, thyme, rosemary, and salt and pepper. Add the chicken breasts and turn to coat well. Cover and refrigerate for at least 4 hours or for up to 8 hours.
Prepare grill for direct grilling over medium-high heat. Oil the grill rack and position it about 6 inches from heat source.
Grill the chicken breasts, turning once, until firm to the touch and opaque throughout, about 5 minutes on each side. Transfer the chicken breasts to a cutting board and cut on the diagonal against the grain with a sharp knife into slices 1/2 inch thick. Capture any carving juices, if possible.
Arrange the chicken slices on a warmed serving platter and drizzle and carving juices over the top. To garnish, sprinkle with rosemary leaves and sprigs and shredded lemon zest, if desired.