Rafael Nadal’s impending retirement will loom over the Davis Cup Final 8 from the moment it begins Tuesday in southern Spain. The blue sign, visible from the highway leaving Malaga’s airport, is marked with large white letters that read, “GRACIAS, RAFA.” Between those two words is an image of Nadal toting a tennis racket in his left hand while wearing one of his trademark sleeveless shirts and his just-as-ever-present headband.