The single most important stroke in tennis by far is the serve: it is the only time you have a chance to hit a ball in hand. There's nothing in the rules saying you have to give yourself an overhead smash on your serve - you could legally hit it underhand, but as it were, the evolution of the game means that the serve is taken when the body can generate the most racquet head speed, imparting the most power, the most spin and the most acute angles.
But what about one serve in particular, that of Novak Djokovic, has made it so good after it was so bad for so long? Years ago, back when he used to lose to the top players more often than he beat them, Djokovic's serve was his albatross. It wasn't the only problem in his game, but it was by far the most glaring. After all, how could a player with such great hand-eye coordination (as evidenced by the second most important shot in the game - the return of serve) be so bad at hitting a ball in hand?
Well, the secret to his success is no secret at all. Like Rafael Nadal, Djokovic hired a coach that worked on his serve and turned it from a liability to an asset. Today, the tactical acumen of the serve, imparted by (who I must begrudgingly admit has done wonders for that stroke and his game in general) none other than Das Wunderkind Boris "Boom Boom" Becker, is as impressive as any other aspect of his game. That's saying a lot, given how good he is as so many other things.
But in order to use the serve effectively from a tactical perspective, it's got to go in - and that's something that he had trouble with back in the day. There are those who bemoan the "lost" year that Novak Djokovic spent with Todd Martin in 2010 as a colossal waste of time. Martin, for his part, has not returned to coaching ATP players, and Djokovic has gone from strength to strength. As such it's easy to dismiss any possible positive impact Martin had on the Djoker's game.
But video doesn't lie.
First, some background: when Marian Vajda was stopped by veteran tennis journalist Ubaldo Scanagatta, in what appears to be an airport lounge in 2011, he dispensed with the stupidity and inadequacy of the "belief" gibberish that Djokovic had been spouting all year about his game, and insisted on a technical explanation for his renewed success, after 2 years of profligacy in the majors. Scanagatta (himself a former University tennis champion in Italy) didn't allow Vajda to perpetuate the ruse, or at least was unsatisfied with it and went shot by shot to discover how Vajda (a mediocre player, but an outstanding coach) transformed his game. In this video, he explained how Djokovic wasn't that far off technically, but among the many issues to be addressed, the serve was chief among them.
Again, few are prepared to give Martin any credit for Djokovic’s serve in 2011, but they worked on that serve for almost a year before it improved. Before Martin, his serve was a disaster (again, not my words, Vajda’s). Don't believe me, or don't remember? Here is the monstrosity that is was in 2009 with the stiff arm, the over-rotation, and a reluctance for his body weight to carry him into the court:
Now there weren't too many people who were able to explain what was wrong with his serve, but it's worth noting that Djokovic didn't address it until he took on Martin as a coach. In this clip, from Indian Wells in 2010, he’s making Djokovic hold two racquets to compel the arm to come straight up to trophy position – without the straight arm:
That solved the problem of the racquet head taking too long to arrive at the point of contact, requiring him to over-rotate. Among the many problems with over-rotation, it typically results in a player not actually watching the ball hit his strings as he serves, as well as putting the momentum of his body straight into the ground following the serve, rather than into the court. Doing so both diminishes the power into the serve and eliminates any reasonable possibility of serving and volleying.
Here, also in 2010 at Indian wells, Martin has Djokovic serve from his knees to compel wrist pronation:
Because he's serving from his knees, he cannot finish with the racquet down at his feet - he'd break it every time. Instead, by shortening the distance to the ground, he compels Djokovic to pronate the wrist after the point of contact, maintaining racquet head speed through the point of contact and allowing him to hit down on the ball. This also alleviates the likelihood of over-rotating, since doing so would land the serve in the ground in front of the net. The wrist pronation not only eliminates any unwitting deceleration prior to the point of contact, it also compels forward momentum into the court.
And finally here is what the serve looked like in 2011 – the stiff arm is almost gone and the racquet head comes almost straight up to trophy position:
As far as the stroke production is concerned, Djokovic’s serve became solid in 2011, just after his parting with Todd Martin. The motion remains largely unchanged, but tactically, he establishes the wide serve in both the deuce and ad courts more now than he did in 2011. He has also incorporated a slice serve "up the T" in the ad court preventing right handed players from sitting on the wide serve and allowing him to shorten the distance past his opponent's point of contact with less risk because he's slicing the serve rather than hitting it flat.
So make no mistake about it - Novak Djokovic didn't suddenly believe in himself, and translate belief into a better serve. With practice and the courage to re-engineer it despite being the 3rd best player in the world at the time, he did it the old fashioned way...