For the men it's definitely Marat Safin. Of the up and coming it has to be Gulbis. He has so much talent but can't seem to get it together.
On the women's side it has to be Vaidisova hands down. She was ranked #8 two years ago when she was 18 years old and has a winning record against most of the top players. But in the last year especially she has sunk in the rankings to #67 and she is only 20 years old. She lost in the first round today to a virtual unknown and hasn't won a title in over a year. What happened?