Why does No. 1 play No. 3 instead of No. 4? A short explanation of seeding in Grand Slam tennis

On Friday, Novak Djokovic, the world’s top-ranked player and the No. 1 seed at the French Open, took on No. 3 seed and seven-time French Open champion Rafael Nadal on the red clay in Paris. (Nadal won in a five-set thriller.) If you don’t follow tennis, but do follow other sports with seeded tournaments, you might assume that this Djokovic-Nadal match is for the championship. In most such tournaments the No. 1 and No. 3 seeds are in opposite halves of the bracket and could not meet until the final game or series. And, had Roger Federer beaten No. 6 Jo-Wilfried Tsonga in the quarterfinals, the other semi would have pit No. 2 Federer against No. 4 David Ferrer. So you would have a situation where the four top seeds advanced to the semifinals, but the semifinals were unbalanced, with No. 1 playing No. 3 in one match and No. 2 playing No. 4 in the other (instead of the No. 1-No. 4, No. 2-No. 3 matchups we’re used to seeing in other sports). Top seed Novak Djokovic faces #3 Rafael Nadal...