I want to just pitch in my 2c. I'm not going to get into the subject of amino acids or glycogen or whatever, because I have limited knowledge in that space. Just two quick things:
1. @Ramon @PowerPlayer, @CDestroyer on the subject of caffeine and b-vitamins: it may be semantics, but I don't believe they energy in the sense that the OP was looking for. They may keep you alert, but there's no useful cellular material to break down and convert into energy. It may change how your body is breaking down its various fuel sources, and it may change your state of mind, but isn't a meaningful fuel source in itself. All things equal, you can't replace food with vitamins/stimulants.
2. I try to follow the pros, even though I'm far from one. I would ask if all things being equal and no dietary restrictions, why would Nadal/Fed/Mahut/Isner and other endurance athletes, choose sports drinks (with sugar?) over drinks that have minimal to no sugar, like Xtend?