Please show this evidence or do you mean GZ's statement makes it evidence since there's nothing to refute it? No one saw how the confrontation started. TM ran, didn't see GZ get out of the car, probably assumed GZ wouldn't and went back to talking on the phone which is well within his rights to do. If all he wanted was to beat GZ's ass, why didn't he do it long before that? He had plenty of opportunities.
How was he going to beat Zimmerman's ass when he was not near Zimmerman;
he was outside Zimmerman's visual range. Past the T.
He needed to intercept Zimmerman; easier to do that also by surprise; that way Zimmerman would not be prepared to fight back.
Usually in military terms is call an ambush from the flank.
You expect that Martin was stupid enough (remember he was in astronaut training) to pull Zimmerman out of a vehicle while Zimmerman was on the phone.
Not knowing who he was on the phone with (or possibly realizing that it was the law because of overhearing) he takes off for safety.
Zimmerman comes along on the phone - Martin's best course of action is to stay out of sight (somewhere).
Zimmerman gets off the phone; Martin has sized him up now; and decides to teach a "respect me class".
There is your best opportunity - minimal risk - maximum impact.