Edit:
After some experiments with bones and constraints I achieved the effect I was going for! It took around 10 minutes to achieve this effect, it’s quite simple to do.
— Pre-Edit
Here it is! I think I overcomplicated this animations, I used a total of 6 bones, I think I can achieve the same effect with two bones and some modifiers, I’ll try that later.
You can do it with one bone!
Keep in mind the 12 animation rules.
One of them is that a deformed object, always keeps its volume.
Stretch means thin in the middle.
Flattened ball is much wider … same volume.
This keep volume constrain is an modifier on the bone set-up.
that’s why you need one bone 
Thanks for your feedback!
I wasn’t trying just to stretch the ball but to add this kind of liquid effect to it, as if the ball was a hollow super elastic rubber ball, that’s why I used several bones, in the end I failed miserably to convey that effect in several ways; the camera angle wasn’t right, the light was way too strong with very little shadow detail, and the effect just wasn’t that good, but I actually achieved it by using only two bones:
There is a feature with Blender, to make the ball soft, like it is a wobbly gelly ball.