From the Super Mario Wiki
Tennis (Singles) is a Court & Field Sports Event that appears exclusively in the Nintendo 3DS version of Mario & Sonic at the London 2012 Olympic Games. The objective of the event is to score 3 points before the opponent, but uses a regular points system as opposed to a tennis one.
In this event, the player plays two matches, a semi-final match against one opponent, and depending on the result of that match, either a final match against the other winner or a third place match against the other loser. At the start of the match, the player's character will always automatically serve the ball after a brief period of time directly to the other character. Once the other character has returned the ball, the player will then be prompted with a direction in which to hit the ball, and the player should hit the ball in that direction by sliding on the touch screen. These arrows remain for the first few shots in the rally, but after a while the player must decide their own direction in which to hit the ball. To score a point, the player must hit the ball past their opponent. However, if the player hits the ball outside of the sidelines, the ball will be declared out, and the other player will score a point. If the player attempts to hit the ball too late, or fails to hit it, they will be told that they hit the ball late and the other character will gain a point. If either character hits a shot with very poor timing, the receiving character will have the chance to perform a special move by sliding in any direction. This will cause them to return an extremely powerful shot which their opponent cannot hit back, and guarantees the character who performed it a point. The match is umpired by Omochao.
1) Forehand: Slide ← on the Touch Screen.