Preheat grill to medium heat and lightly oil the grate.
Rub tilapia fillets with 2 tablespoons lime juice and season with salt, black pepper, and chili powder. Brush both sides of each fillet with olive oil.
Grill tilapia on preheated grill until fish flakes easily with a fork, about 5 minutes on each side.
Heat tortillas on an ungreased skillet over medium-high heat for 10 to 15 seconds. Turn and heat for another 5 to 10 seconds. Fill the heated tortillas with flaked tilapia. Top with tomatillo salsa, avocado, and sour cream. Sprinkle with cilantro and 1 tablespoon lime juice.