I don't know what federer is for sure, but it is very hard to spot an introvert unless you are one or the person is painfully obvious about it. It becomes even more difficult the further away on the introvert/extrovert spectrum you are.
Extreme extroverts tend to think of introverts only as being shy and reserved people (think Sampras).
Moderate people are able to better identify both sides of the spectrum.
Introverts often can better identify introverts due to their understanding of their own tendencies. People unlike them often appear to just be extroverts.
For instance, an extreme extrovert envisions his ideal time involving social interactions. A typical introvert could feel the same way, but it would probably be more specific to a small number of people. Extreme introverts may only interact socially because they know they should or have to (the world has become very biased toward extrovert behavior). These same people would probably find more joy in spending a lot of time alone.
Personally, I interact well with others because I have learned to do so. Deep down, it is painful to interact with new people and stressful to be in medium to large sized groups. I don't like communicating my thoughts and opinions except to very close friends, but I have no problem rambling on and on about things I am excited about when I feel like people are truly interested. I could probably spend years with only another person or two and be perfectly happy.
I personally see federer in as fairly moderate or slightly introverted. It is almost impossible to tell because he has been in the spotlight for so long that he has learned a specific way to interact and present himself. Fans rarely see him in his typical state of behavior because he is always busy with tennis.
The power of introverts ted talk for anyone interested : http://www.youtube.com/watch?v=c0KYU2j0TM4