From the Super Mario Wiki
Wonder Flower is Daisy's Offensive Power Shot in Mario Power Tennis. Daisy's racket grows petals and spins around. This causes the ball to fly at normal speed, masking power it hides and, after contact, knocks the opponent back about an eighth of the court. It has a decent curve and its recovery time is one of the best in the game. On a side note, as with most other Offensive Power Shots. it causes the opponent to lose whatever Power Shots he or she may have been storing. The flowers also have the side effect of making the player hit uncontrollably rush to the net.