My first Dreamcast's GD-ROM laser was fine but the motor that was responsible for spinning the disc went out. The brushes inside literally burned down to nothing. Dreamcasts do NOT like the slightest scratches on the game discs. Those units were made by Yamaha and later by Samsung. It might be possible, if the motor is at fault, to just replace the motor with an equivalent. That's what I did to fix mine. But, Radio Shack is not around any more so finding that motor might be an issue.
Sega use to have replacements long after they stopped productions of the units. You might try contacting them to see. It couldn't hurt.
The only other thing I could suggest, and I don't approve of piracy, but get the ISO of your Virtua Tennis from ISO Zone and burn it to a CD-ROM (don't use blank music discs designed for music! They don't work most of the time). You will have a scratch-free copy that you can test with. Dreamcasts will read CD-ROMs without any modification required.
I can also extend my repairing know-how with you and offer my services. If you continue to have issues with your DC, and are comfortable with paying for shipping both ways, I will do what I can to fix your DC console. I would only need the console has I have two here and all necessary hook-ups to operate the system.