I saw George Will on TV and he used a word (and immediately defined it) but all I was heard was the definition, not the actual word. "Emmetropic" is the closest I can come to finding a word that meets the definition "simultaneously relaxed and focused" but it seems to have such a specific, physical, meaning and the context seemed to me to indicate a mental state rather than a physical one.
Try in the zone
1. In a mental state of focused concentration on the performance of an activity, in which one dissociates oneself from distracting or irrelevant aspects of one's environment.
Here's another definition
a temporary state of heightened concentration experienced by a performing athlete that enables peak performance (i.e. players in the zone)
This is probably jargon, but "flow" seems to describe what you mean.
Don't interrupt him, he's in the flow.
Zen is a word used to describe a relaxed and focused meditative state.
I do my best work when I am in a state of Zen.