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Well, if you're more elaborate on what you mean. There could be a simple reason such as, you're really focusing. When I focus hard during exercise I can't hear anything and I only hear my breathing, my stepping and even sometimes my heart rate.

Since you say, when you run a lot during tennis, this must mean somewhere in the peak of a set when all the difficult shots are being returned. It's all normal, don't worry. Unless if you mean you're not even focusing and you just can't seem to hear than perhaps you should talk to your GP.

I say it's due to focus, so don't stress, means you're trying real hard.

It could be Meniere's disease. Meniere's can cause hearing loss. Because it is caused by the build up of salt crystals in your inner ear, you could be moving the tiny crystals around when you play tennis, thus causing a momentary hearing loss. That's one possible reason for your temporary hearing loss. If it is the cause, you should eat foods that help to lower your salt count, such as bananas. These kinds of foods also help to keep you hydrated for sports.

I myself suffered from hearing loss, but it was never exercise related. I've never heard of anything like that. The best advice I can give is to first consult your physician and maybe and otolaryngologist a.k.a Ear nose and throat specialist.