Marinate the chicken by combining olive oil, lemon juice, minced garlic, salt, pepper, and chili powder in a bowl. Add chicken breasts and coat well. Cover and refrigerate for at least 30 minutes.
Prepare the avocados by cutting them in half, removing the pit, and slicing or mashing them. Drizzle with lemon juice to prevent browning.
Preheat the grill to medium-high heat. Grill the marinated chicken for about 6-7 minutes on each side until it reaches an internal temperature of 165°F.
Assemble the dish by placing grilled chicken on a platter and arranging avocado slices on top or beside it. Optionally, sprinkle with cilantro and drizzle with lemon juice or olive oil.