I definitely think Barthel will be a lot more tricky than Venus. Venus isn't able to be consistent enough off the ground to match Vika anymore, whereas Barthel can occasionally string together good patches of play (like in Stuttgart and AO) which could be a lot more difficult if Vika is feeling sloppy.
I WAS SO WRONG. Venus played her best match of the year by FAR. Should be an interesting match if Venus plays like she did today.