This happens to me a lot, and I have eczema, which tends to worsen the condition. Even when you're not playing tennis, just overnight you could also try applying Vaseline on the crack and wearing a cotton glove (not sure where you actually would get the cotton gloves but I think mine came with my case of Vaseline) to help it heal.
Great advice!
Cotton gloves with cream or vaseline will help.
The cotton gloves are sold at any drugstore or mom and pop pharmacy. As the weather gets colder and dryer, this problem can worsen. So, ot only for the cracking but for sinus and respiratory health,
during the winter you should use a cool mist humidifer.
I had this same problem for 20 years and not just on my thumb because I also swimmed on a daily basis and the chlorine magnified the problem.
The key with the cracking and/or eczema is to keep the area dry. I stopped swimming and
the condition went away with the following additional steps!
My internist who had the same problem advised me to
wear surgical latex gloves without powder when showering held with rubber bands on my wrist so as to not get any water on my hands. In addition, I was advised to and used used
Cetaphil Daily Cleanser as my hand soap rather than regular soap. Cetaphil can be used with or without water to clean/ wash your hands. When the cracking is bad, you can use the cetaphil without water. Apply the cetaphil to your hands and rub them together. Remove the excess with a paper towel. Instructions are on the bottle. Cetaphil also acts as a softener/moisturizer.
After a very short time, the cracking disappeared. It has never really returned.
I now use cetaphil as my regular hand soap with water. At the first sign of cracking, I use the cetaphil without soap. However, I have never had the problem again.