By Steve Keating MIAMI (Reuters) – While Roger Federer and Rafa Nadal may be the most trendy rivalry in men's tennis, the most enduring may be top ranked Nadal and Novak Djokovic, who clashed for the 40th time on Sunday in the final of the Sony Open. Any final featuring the world's two top ranked players automatically qualifies as significant but Nadal and Djokovic have elevated their clashes to something special. Despite Sunday's loss, Nadal continues to hold a slight edge, 22-18, in their head-to-head meetings, 20 of the matches having come with a title on the line, including six grand slam finals. With Djokovic's Miami victory, he and Nadal between them now hold all nine Masters series titles.