When you become Number 1 Pro in the world you get a message from Tim Henman saying Congrats - you should then automatically unlock all Sega gear. (This is when I got it)
You will need to then go to the Tennis Store and purchase:
- White SEGA T-Shirt
- Blue SEGA Shorts
- Blue SEGA Cap
- Blue/White SEGA Trainers
- Black SEGA Sunglasses.
Change all your gear back at home, play any match and achievement unlocked.