No two modern-day players have met in competition as frequently as Rafael Nadal and Novak Djokovic, but when the Spaniard and the Serb square up here on Friday in the Wimbledon semi-finals for the 52nd time there might be plenty that the two men feel they do not know about each other. With Djokovic playing in his first Grand Slam semi-final for nearly two years, Nadal will be wondering how close his rival is to recapturing the form that brought him 12 Grand Slam titles. Djokovic, meanwhile, might be asking himself whether Nadal still has it in him to complete a French Open-Wimbledon double, which he last achieved eight years ago.