Undergrip for Tennis Problem?

I have a black undergrip and I tend to put stress on all parts of my hand when I play tennis. As time goes by I notice that I have a lot of callus' even though I have been playing tennis for quite some time and my hand is as rough as sandpaper. I also notice that my hand is like super black as well and it hurts after 2 hours of play. I don't like overgrips because it creates thicker dimensions onto my grip which I don't prefer because I like my tennis handles to be relatively small. Is there anyway to avoid this or any overgrips you have in mind that are really thin?