There's a fine line regarding your question. At first it appears like a "no brainer" and of course you need to focus more to play better. Like many things at the deeper level this could be counterintuitive.
When playing "in the zone" or "in dead stroke" there is no conscious attempt to focus or not focus, concentrate or not concentrate. In the study of Zen it's suggested to become absorbed into what you are doing without regard to the conscious mind.
From my experience, the player and Game must become one to the level you are unaware whether you are playing the Game, or the Game is playing you..... 'The Game is the Teacher' CJ Wiley
My definition for "Being in the Zone" means:
1. You won your 1st game, 2nd, and no pressure any more, you do not hear people say "He is in the zone when he looses a match, it seems this phrase is reserved for winners only
2. You are holding cue butt lightly, your wrist is fluid, you are following through and staying down on almost every shot therefore, your CB speed is almost perfect due to muscle memory.
3. You know every little secret about the game advance knowledge as well as applying it, and you figured out the table cloth well. Additionally, most shots on table seem to align with almost all your check list items that "you are aware of". (Some check list items, we are not aware that effect a shot which is major reason for us not being consistent at times).
4. You are at your favorite pool hall, playing your best "fish", on the table you love the most, favorite CB, and game you like, surrounded by your best fans
5. No outside distraction, no guilt feelings, you taken care of work, family and everyone you deal with; health is good, slept very well last night, happy about life at least that day, all of these are
BIG factors in freeing your memory bank which in turn and automatically allocates freed memory to what you are doing weather pool, play music, play chess, giving a speech..etc. Having free memory in the conscious mind helps in decisions making so you select better patterns more often, which way to shoot some shots, and seem to help on keeping your winning $$, or match and not let it all go away.
6. Sorry, luck on your side more % than opponent; balls seem to favor you more " always luck sides with winners" (you cannot be lucky and loose)
7. If you cannot "Be in the Zone" all the times or 99% of times, back to the drawing board and start figure out what did you do wrong, which check list item missed, when exactly things started to go wrong, keep record of it, and trust your inner feeling, and go out and prove what you came up with in money game, if it works great, if not keep trying..
8. Other issues i am sure..
Best of luck