After dominating tennis so completely in his mid-20s, Roger Federer has only one trajectory remaining in a brilliant career: downward, as certain as gravity. And even that has been graceful. It’s difficult to pinpoint a skill of Federer’s that has diminished these last two years, in which he slid from No. 1 in the world to No. 3. His movement, at 30, remains fleet and fluid. His serve is shrewdly placed with plenty of pace. And he brandishes a full array of strokes and the smarts to deploy each as circumstances dictate.