old lilac tree issue

Hi, we have an old lilac tree in the garden, where the leaves are really only on the ends of the branches, so inside the bush it is quite bare and 'twiggy'. It has been previously cut in to a triangular shape resembling more of a hedge than a shrub! I'd like to cut it right back ideally into a ball shape or certainly a smaller triangle, but am worried we will end up with just sticks! any help, advice or suggesions for an ametuer greatly appreciated! Thanks.