NEW YORK—Through a third-story window over Broadway you could have seen him, the reigning light heavyweight champion, all 6 feet 4 and 205 pounds of him, twisting his torso, unleashing his kicks. It was after 11 p.m., and the Equinox gym off 19th Street had sent its regulars home, but Jon Jones was still sweating, dressed in red shorts and a black hooded sweatshirt, finishing a final workout before leaving New York for his Ultimate Fighting Championship title defense Saturday in Atlanta.

Jones is 24. He is perhaps the brightest...