A round robin is a grouping of players in which each player must play every other player in the group. The format can be used with any number of participants over two. According to the U.S. Tennis Association, when six or more players are entered in a tournament, they may be divided into multiple round-robin groups. For example, a tennis tournament might have eight participants divided into two groups of four for round-robin play. That means each player must play everyone else in her group once, so each entrant will get to play at least three matches. In a single-elimination format, a player is eliminated if he loses, so half the players are out after playing only one match, and all but the finalists play fewer than the three matches guaranteed by the round-robin format.
If the tournament is a round robin with only one group, the player with the best match record wins. If there are multiple round-robin groups, the players with the best records in their groups advance, often into an elimination format. There are different ways to do this. The simplest way is for the top player in each group to advance to a final match to determine the tournament champion. Another way is for the the winner and runner-up from each group to advance into the elimination format. In that case, the player with the best record in Group A would face the player with the second-best record in Group B in one semifinal, and Group B's top player would play Group A's runner-up in the other semifinal. The semifinal winners would meet in the championship match.
The player who wins the most matches wins the round-robin group. But what if two players are tied with the most match victories? According to the U.S. Tennis Association, the winner of their head-to-head match wins. If three or more players are tied, the referee or tournament director ranks players using the following criteria until no ties remain: head-to-head records in matches involving just the tied players; percentage of sets won among all sets completed; head-to-head records among remaining tied players; highest percentage of games won. If a tie remains, a random drawing determines the order of finish among remaining tied players.
In tournaments with multiple round-robin groups, USTA rules require the tournament director to rank, or seed, the players so the strongest players start out in different groups. For example, the top-seeded player would begin in Group A, the second-seeded player would be in Group B, the third-seeded player in Group C until every group has at least one seeded player.
In professional tennis, the same basic round-robin rules generally apply. Most professional tournaments are strictly single-elimination events, but both the ATP (men's tour) and WTA (women's tour) year-end championships use round robins for the preliminary stage. The eight highest-rated players of the year compete in two four-person round-robin groups, and the winner and runner-up from each group advance to the semifinals.