This comes from the Iwata Asks interview...
Iwata: What difficulties were involved in making Tennis online?
Makino: In a typical online game, the information of the button presses are sent to your opponent, and the inputs are synchronized between the players. But for this game you have to send complex information from the gyro sensor and accelerometer of the Wii Remote Plus controllers, in addition to information from the buttons, which was a brand new challenge.
Iwata: And in Tennis, there may be four people on the court at once.
Makino: Yes. At first, we were uneasy about whether we could really do it, but we tossed around all kinds of ideas with Namco Bandai Studio.
Shimamura: We then decided to test some of the ideas, and the one we eventually adopted was an idea suggested by a communications programmer at Namco Bandai Studio. Thanks to that, we were able to pull it off.
Source: Iwata Asks