I brought up this topic, cause Berger's serve looked unusual but he still got the job done reasonable well.
Similarly people talk about how strange Jim Furyk golf swing's compare to the other classic golf swings but he gets the job done and wins too.
The thing I find unusual in Berger's serve is he lacks a windup with his racquet arm yet still manage to get good pace on his serve.
I believe he gets that pace by his natural rhythm
He gets weight shift and momentum as you can see he finishes into the court after he serves.
In general I have observed the deeper one lands into the court with the foot after striking the ball the more pace the ball has if cleanly struke.