"Virtua Tennis & Tennis 2K2
Having the essential quality of being easy to play and difficulty to master, Virtua Tennis was one of the sleeper hits on the Dreamcast. It is often overlooked as a killer party game — I have had many parties with both guys and girls of all gaming levels having a blast on a 4-player doubles tournament.
While other enjoyable tennis titles have come long since (Top Spin and Mario Power Tennis), Virtua Tennis still holds up as being possibly the best tennis game ever. The “sequel”, Tennis 2K2 is pretty much the same as Virtua Tennis except that the graphics are a bit more polished and it adds to the player lineup with a female roster (including the Williams sisters).
Not only is Virtua Tennis a great multiplayer game, but it also features an entertaining Soul Calibur-style single player game, where the player must progress through the ranks to become the world’s top player. Even if you don’t like tennis too much, this is still an essential game, and an example of multiplayer Dreamcast gaming at its best."
Also, here's an old thread on the topic I saw in google

viewtopic.php?f=1&t=9701