I didn't know much about Novak Djokovic until recently, but in this article, an interesting fact is revealed about him: since he's never been a fan favourite (not nearly on the level of Federer and Nadal), he's always had to fight extra hard to win his matches, as the crowds were against him more often than not.
In spite of that, he's one the verge of becoming the greatest player of all-time. Indeed, if he wins one more major title, he'll have won more Grand Slam titles than any other player in history, among a myriad of other stats that he's already the all-time leader for.
While I'm not especially fond of tennis (the truth is that in spite of years of practice as a kid, I still can't hit a ball properly), this brings up two interesting ideas to mind:
The first is that he's become arguably the greatest player of all time (or at least in the Open Era) without fanfare, slowly and steadily making his way to the top, while most tennis fans were busy fawning over Federer and Nadal, who were themselves too busy poorly acting in unmemorable commercials. Another empirical piece of evidence that consistency is the most important key to success.
The second concerns the sheer mental fortitude it takes for someone to win so thoroughly when the crowd is mostly against you, and sometimes quite vocal about it. It shows an incredible amount of self-confidence and strength of character.
Not everyone has the chops to be the Djokovic of their field, but these lessons are nonetheless applicable to any endeavour: to succeed, even at a moderate level takes huge amounts of dedication and fortitude.