In fact, it is common practice for players to get the rackets they play with custom-painted to look just like the racket they're being asked to promote!
I haven't heard of any cases where a player is using a completely different brand of racket, but it is quite common for them to stick with an older model (which may or may not still be available to the public) or even use a custom racket that has been specifically designed for them by the manufacturer.
Tennis racket manufacturers are well aware of this practice which has been going on for years, and tacitly approve it by contracting with players in such a way that they are not legally obliged to use the latest racket, but merely to "give it a try" at some point. Whether that's in a match or just on the practice court is unclear. Of course, we're not just talking about Wilson tennis rackets here, but also those from all the other big manufacturers - Prince, Head, Babolat and so on.
Why racket paintjobs?
The reasons for painting rackets are fairly obvious when you think about it:
- Manufacturers know that customers are more likely to buy a particular racket if a well-known champion uses it, even if customers rationally understand that doing so is unlikely to make much of a difference to their own game.
- Players are loath to change a racket they're comfortable with and that has brought them success in the past. Even if another racket is equivalent or even superior, sportspeople are notoriously superstitious about such things - some won't even step on the lines between points!
The logical conclusion is to make the general public think a player is using a particular racket, even if he or she isn't.
The ethics of racket painting
Mark Philippoussis played with an entirely black racket because he used rackets that were no longer available to the public and wasn't prepared to participate in the deception, but most other players seem to have less of a problem with it - and not surprisingly considering the amounts of money on offer from the manufacturers.
Back in 2000 there was a similar situation in golf, where Nike Golf was sued by a consumer watchdog group after Tiger Woods disclosed that the sponsored balls he used were not available to the general public. Shortly after the suit was filed, Nike made the Tiger ball generally available - an admission on their part that they knew there was substance to the lawsuit. As yet there have been no similar cases in the tennis market that I'm aware of, but perhaps it is only a matter of time.
Does it matter?
Clearly there is an element of deception here. Whether or not it makes a material difference to the consumer is something for you to decide.