It is almost impossible to find a suitable opponent for Maria. Just look at her heigt and fitness level. I think Franziska van Almsick is the only one posing a challenge to her. I choose her.
Another suggestion would be Caroline Woczniacki. Although shorter than Maria she is currently on the top of her shape. She could be a real challenge.
WHoever will fight Maria, it will become an interesting one I am already looking forward to!!
