You can currently delete only the first or last endpoint of an object. For the circle, I drew an ellipse and then pressed Cmd-8 to Convert to Path. Then, select the object and then the pen tool. You'll see that the cursor has a little minus sign next to it when it's hovering over this endpoint. Then, you can continue drawing (as it opens the path), or not. In this case, I've Cmd-clicked an endpoint and then pressed Escape to give me my 3/4 circle. (Note that I left the fill on in order to show you where the points are.
Hope that helps!
Awesome, this works!
Thank you so much for your answer
PS: Please include gestures as swiping etc. as soon as you can for Xd (I already voted for it) -then its officially the easiest and best tool for UI/UX!
You're welcome, and thanks for voting for the gestures story!