Bad left knee.... no problem.
Stand much more back to target.
Pivot off your bad left leg and LAND on your good right leg.
BrianGottfried served like this, as did a few other guys, maybe Becker.
The pivot affect gives you twist power, so you don't need to jump as high, or not jump at all.
I find, at 63 now, I can serve full speed flat serves without jumping, around 100mph, because by not jumping, I can serve with more precision.
Don't jump. Instead, pivot and twist into your serves.