A few years ago, Steven R. Selsberg, a partner in Mayer Brown in Houston, thought he would make a smoking-hot career out of tobacco litigation, but today Selsberg is the go-to litigation lawyer in the United States for the business empire of Carlos Slim Helu, the Mexican billionaire who may be the richest man in the world.

In August, Fortune magazine pegged Slim’s net worth at $59 billion, based on the value of his public holdings as of the end of July.