bottom four players may play a two-round tournament, the winner of which plays the top player The bottom two players may meet, after which each subsequent game pairs the winner of the previous game with the next player However, the number of arrangements grows quickly for larger numbers of players and not all of them are commonly used. Seeding Opponents may be allocated randomly (such as in the FA Cup); however, since the "luck of the draw" may result in the highest-rated competitors being scheduled to face each other early in the competition, seeding is often used to prevent this. Brackets are set up so that the top two seeds could not possibly meet until the final round (should both advance that far), none of the top four can meet prior to the semifinals, and so on. If no seeding is used, the tournament is called a random knockout tournament. Standard seeding pairs the highest and lowest, then second highest and second lowest and so on, for an 8 seed tournament this is 1 v 8, 2 v 7, 3 v 6 and 4 v 5, for example this is used for 16 seeds in the World Snooker Championship and 32 seeds in the World Darts Championship. Some tournaments stray from this, for example is not the procedure that is followed in most tennis tournaments, where the 1 and 2 seeds are placed in separate brackets, but then the 3 and 4 seeds are assigned to their brackets randomly, and so too are seeds 5 through 8, and so on. This may result in some brackets consisting of stronger players than other brackets, and since only the top 32 players of 128 are seeded in Tennis Grand Slam tournaments, it is conceivable that the 33rd-best player in a 128-player field could end up playing the top seed in the first round. An example of this occurring was when World No. 33 Florian Mayer was drawn against (and eventually defeated by) then-World No. 1 Novak Djokovic in the first round of the 2013 Wimbledon Championships, in what was also a rematch of a quarterfinal from the previous year. Sometimes the remaining competitors in a single-elimination tournament will be "re-seeded" so that the highest surviving seed is made to play the lowest surviving seed in the next round, the second-highest plays the second-lowest, etc. This may be done after each round, or