Word that means "Simultaneously relaxed and focused"?

Try in the zone

adjective
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.

It sounds almost like he was talking about hypnosis. I think the hypnagogic state is one in which you are both relaxed and focused.

But it also might have been one of my favorite words, excogitation, which means to ponder or think intently about something. I love that word.