Roddick feet

one of the issues that you must resolve is the spread of your feet on the serve Rodick, Thiem, Chung had their feet very close together on their balance

low volley

on the low volley get the back knee below the flight path and slightly open the the racquet face and carry the follow through up to net level

spring up

your legs are a major power source– you ideally want to grow your serve so that you spring into the hit