"The Sports Gene" [1] is an excellent book on this topic backed up by not only research but entertaining stories as well. It has a section about Dan but also has stories about a world champion high jumper who dedicated his life to high jumping only to be surpassed by someone who wasn't really interested in it and had been practicing for less than a year.

I think the lesson to take away from Dan's experience and what the author of this book argues as well is that yes, practice can make you a lot better. But to be among the best in the world, you'll need to have some sort of genetic advantage.

[1] https://www.amazon.ca/Sports-Gene-Extraordinary-Athletic-Per...

Not in professional sports, see http://www.amazon.com/Faster-Higher-Stronger-Generation-Supe... or http://www.amazon.com/Sports-Gene-Extraordinary-Athletic-Per... (similar ideas in both books)