I don't know if the term flow came before in the zone, but it has been in use for decades. Csikszentmihalyi published a book called Beyond Boredom and Anxiety: Experiencing Flow in Work and Play in 1975.
Tennis players speak reverently of the mystical atmospheric condition known as ‘The Zone’ Passing shots chip away at the lines, first serves pop in and mistakes simply don't materialize.
The next citation is from 1990, and refers to the more general creative sense.
Performers, surgeons, or creative artists..find that most important parts of their lives occur when they are ‘in the zone’.
Source is the New Age Journal. Notice how it is no longer capitalised.