Monfils is like those young american basketball players who are able to dominate the high-school leagues due to sheer athleticism, but can't make the transition to the NBA properly because they lack foundations and brains.
The transition was ok, he went up to the top50 quite easily. But from there, he just stopped improving.
It's totally different from Basketball I would say. Most junior player don't make the transition right, because of their lack of athletism and power when they have to face pro (see D.Young). Serve by instance becomes much more important at the pro level than at junior level.