It happens quite often in tennis, the young stud in juniors who is labeled as the next big thing to hit the ATP tour.
Sometimes, they pan out but not often. Most likely, the youngster gets to the tour with too big a head or is overwhelmed. They may go on to have decent careers but are nothing special.
There have been quite a few guys over the past decade who fit this bill.
It's really hard to label the underachievers over the past decade because of the dominance of Nadal and Federer, but all these guys could have had better or can have better careers.