Synonym for "focused"
You might try intent or fixated; also, a word that seems to capture being in the zone and enjoying it: rapt.
- Riveted
- Centered
- Attentive
- Absorbed
- Engrossed
- Engaged
- Occupied
A one word (adjective), but depending on the context which is hard to judge, maybe acute, sharp or fixated. Applied to martial arts a common two words combination is "intense/impeccable concentration/focus".
What about concentrated? I think it works.