Come on mate, this grandpa is gonna show you whose ur daddy!"
when did you morph into an east-end thug Boon...?
I'm always wary of applying "real uses in situation x y z" to ki principles, mainly because my own view of using ki within my practice is more akin to correct visualisation and mental posture rather than some nebulous energy I've managed to tap into.
However, within the confines of my own take on ki, no I don't have to be in touch with the ground to "extend ki", but I do have to be centered within myself so if the large attacker who's just used me like a rag doll has taken that away from me - no I couldn't use ki (or probably anything other than spastic flailing).
Personally, I find the ideas promoted by training as though Ki exists more helpful than trying to calculate the correct vectors on the fly or some other more mechanical approach, but whatever works for you is good.