In a single-round-robin tournament, 10 chess players are participating. After what minimum number of rounds can it happen that a sole winner has already been determined prematurely? (In each round, the participants are divided into pairs. Win - 1 point, draw - 0.5 points, loss - 0).
Evaluation. After the sixth round, 30 points have been played, and the leader has no more than 6 points, while the other nine participants have collectively scored no less than 24 points. Therefore, among them, there is at least one who has no less than 3 points. Since there are still 3 rounds ahead, the winner is still unknown.

Example. Suppose in the first seven rounds, the leader won all their games, and the other games ended in draws. Then, two chess players who have not yet played against the leader have 3.5 points each, while the others have 3 points each. Since there are 2 rounds left until the end of the tournament, the winner is already determined.

## Answer

After 7 rounds.