When tennis players win six games each, they generally play a tiebreaker to decide the winner of the set. (Tennis rules governing tiebreakers differ for team tennis and for the Wimbledon Championship and may differ for local tournaments as well.) If you're playing a tiebreaker, follow these steps:
The player due to serve the 13th game serves the first point into her opponent's deuce court.
After the first serve, the serve goes over to the other player, who then serves the next two points, serving first into the ad court and then into the deuce court.
After the third point, the players alternate, serving two points each — always beginning on the ad court.
After the first six points are played, no matter what the score is, the players change ends (and continue to do so after every other six-point span) until one player wins at least seven points with a margin of two points.
The player who serves the last game of the set before the tiebreaker serves the first game of the new set following the tiebreaker.