Gather all ingredients.
Preheat a grill to high (450°F to 500°F). Combine tomatoes, red onion, basil, serrano chile, vinegar, ¾ cup olive oil, 2 teaspoons salt, and ½ teaspoon black pepper in a glass or ceramic 13- x 9-inch baking dish. Let stand at room temperature while preparing chicken.
Brush chicken with remaining 1 tablespoon oil; season with garlic powder, Italian seasoning, and remaining 1 ½ teaspoons salt and ½ teaspoon black pepper. Place chicken on oiled grates; grill, uncovered, until an instant-read thermometer inserted into thickest portion of chicken registers 160°F, 3 to 4 minutes per side.
Nestle cooked chicken into dish with tomato mixture; spoon juices over chicken. Let stand at room temperature 15 to 30 minutes. Garnish with more fresh basil; serve with crusty bread.
