The long arc of a professional tennis player’s career does not ordinarily conclude like a fairy tale. If other athletes, particularly those in team sports, might find a way to cheat time — take LeBron James — the tennis player more often tries to defy it, toiling through their twilight years on the circuit in a body that doesn’t quite work the way it once did. Even Roger Federer, you might recall, was bageled in his last-ever Grand Slam match, while Serena Williams...