As Ilya's post suggests, then-Judge John Roberts was not the first to compare the role of a judge to the role of baseball umpire. Indeed, the comparison probably goes back to the invention of baseball, and at the very least has been around for many decades. I'm reminded of one notable usage, Justice Jackson's description in 1951 of how Second Circuit judges Learned and Gus Hand approached the job of deciding cases:

[Learned and Augustus Hand] have represented an independent and intellectually honest judiciary at its best. And the test of an independent judiciary is a simple one—the one you would apply in choosing an umpire for a baseball game. What do you ask of him? You do not ask that he shall never make a mistake or always agree with you, or always support the home team. You want an umpire who calls them as he sees them. And that is what the profession has admired in the Hands.