The following are known Action Replay Codes for Virtua Tennis on SEGA Dreamcast (Dreamcast). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

P1 win one point to win a game (com will not hit a ball served to them)
E1C714A8
00000003

Infinite Cash for World Circuit
9488C0FB
0098967F

Have all characters (KING and MASTER are hidden off the sides of the screen)
9DA80AD7
00000001
E39718B4
00000001
168416B1
00000001
626B6B61
00000001
9E680AD7
00000001
E05718B4
00000001
3F112467
00000001
8B038D20
00000001
B0A63AAD
00000001
16F416B1
00000001

Com can not score any points
4FFA13F5
00000003