Draw a rectangle for the bottle's neck, using the Rectangle tool (M). Center this shape with the others. Switch back to the Ellipse tool and draw a thin ellipse for the rounded bottle top. Make sure the edges of ellipse match up with the edges of the small rectangle. It can help to use Smart Guides for things like this. Turn them on under the View menu. When the points intersect, the word "Intersect" will display. Smart, eh?