Combine tequila, lemon juice, orange juice, chili powder, jalapeño, garlic, salt, and pepper in a large bowl. Add the chicken breasts. Refrigerate and leave to marinate overnight.
Heat a grilling pan or BBQ grill. Brush the grill with oil to prevent the chicken from sticking. Remove the chicken breasts from the marinade, sprinkle well with salt and pepper, and grill starting with skin down until golden brown, for about 5 minutes. Turn chicken over and cook for another 10 minutes, until cooked through.
Remove from the grill and place on a serving plate. Cover tightly with foil and let sit for 5 minutes. Serve hot or at room temperature.