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A single-elimination tournament, also called a knockout tournament, is a common method used to determine the champion in a wide variety of competitive, particularly athletic, contests.
The term connotes the idea that one defeat eliminates the contestant or team from winning the championship or first prize in the event; it does not, however, always mean that the defeated competitor will not participate further in the tournament, as in some such tournaments, consolation or "classification" contests are subsequently held among those already defeated to determine the awarding of lesser places.
Whenever possible, the number of participants in a single-elimination tournament is a power of two. This tournament format enables a relatively large number of competitors to participate; for example, the Wimbledon singles championships are tournaments of 128 players.
The competitors are paired off, one round at a time, after which the number still eligible to win the championship is reduced by half. The round in which only eight remain at the start is generally called the quarterfinal round; this is followed by the semifinal round in which only four are left, the two winners of which then meet in the final or championship round.
When matches are held to determine places or prizes lower than first and second (the loser of the final-round match gaining the latter position), these typically include a match between the losers of the semifinal matches, the winner therein placing third and the loser fourth; sometimes contests are also held among the losers of the quarterfinal matches to determine fifth through eighth places (this is most commonly encountered in the Olympic Games). In one scenario, two "consolation semifinal" matches may be conducted, with the winners of these then facing off to determine fifth and sixth places and the losers playing for seventh and eighth; or some method of ranking the four quarterfinal losers might be employed, in which case only one round of additional matches would be held among them, the two highest-ranked therein then playing for fifth and sixth places and the two lowest for seventh and eighth.
In cases where the number of competitive entities at the start of the tournament is not a power of two, one or more competitors may receive a bye in the first round, which entitles these competitors to advance to the second round automatically without playing. Most of the time, these byes will be awarded to the highest-rated competitors in the event as a reward for some previous accomplishment; indeed, in some American team sports - most notably football - the number of teams qualifying for the postseason tournament will be intentionally set at a number which is not a power of two, in order to provide such an advantage to a high-achieving team in the just-completed regular season.
Another key element in single-elimination tournaments is known as seeding. This is a technique that ensures that the highest-rated competitors will not be scheduled to play against one another until late in the tournament. In conjunction with seeding, brackets are set up, so that the top two seeds could not possibly meet until the final round (should either or both advance that far), none of the top four can meet prior to the semifinals, and so on. Ideally, the brackets would be set up so that the quarterfinal pairings (barring any upsets) would be the 1 seed vs. the 8 seed, 2 vs. 7, 3 vs. 6 and 4 vs. 5; however, this 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 16 players are seeded at all in most tournaments, it is conceivable that the 17th-best player in a 128-player field could end up playing the top seed in the first round. While this may seem unfair to a casual observer, it should be pointed out that rankings of tennis players are generated by computers, and players tend to change ranking positions very gradually, so that a more equitable method of determining the pairings might result in many of the same head-to-head matchups being repeated over and over again in successive tournaments. These scenarios form the origin of the common phrase "luck of the draw."
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 only at selected intervals. In American team sports, for example, both the NFL and NHL employ this tactic, but the NBA does not (and neither does the NCAA college basketball tournament). The NBA's format calls for the winner of the first-round series between the first and eighth seeds (within each of the two conferences the league has) to face the winner of the first-round series between the fourth and fifth seeds in the next round, even if one or more of the top three seeds had been upset in their first-round series; critics have claimed that this gives a team fighting for the fifth and sixth seeding positions near the end of the regular season an incentive to tank (deliberately lose) games, so as to finish sixth and thus avoid a possible matchup with the top seed until one round later.
Compare double-elimination tournament
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