I'm currently writing an article in which I connect two open problems from different fields of mathematics (group theory and combinatorics). While writing the introduction, I was trying to think of other examples of two problems from different fields of mathematics that are connected.
I could only think of the modularity theorem (formerly known as the Taniyama–Shimura–Weil conjecture) and Fermat's Last Theorem. Both are now solved, but historically, it was first shown that they are related; more specifically, that a negative answer to Fermat's Last Theorem would yield a negative answer to the Taniyama-Shimura-Weil conjecture.
My question is: What are other examples of such connections?
Edit: based on coLaidernotte's suggestion, the type of connections I'm looking for can better be described as follows: examples of two (or more) open problems that are not considered related at first, but which are eventually proved in a uniform way or proved to be closely related.