Nadal has a very unconventional forehand that uses his bicep more than the conventional forehand. So yes, his left bicep was developed from playing tennis. Not exactly the fastest way to build your 'ceps, but if you're playing tennis 6 hours a day using that stroke, you'll see some results over the years.
Biceps are useful for tennis, and they look good, so it's a win-win. They shouldn't be the focal point of your routine, nor should they be completely neglected.