Looking at the current world rankings, Nadal is 12500 pts, Djokovic is at 12330. Then there is a precipitous drop in points, with Wawrinka at 5525, Federer at 4945. Why would there be such a drop? I attribute this to the simple fact the prior two have more topspin and sidespin on their forehands.
Let's be honest here. The serve is no longer the determinant of success on tour. If it were, Raonic and Isner would be in the top five. And the other matter is that more U.S. men are neglecting what should be obvious. To win consistently above the rest of the field, one needs a versatile topspin forehand.
The men's tour used to be mesmerized by Federer and his high RPM forehand. But then Nadal came along with the capability to nearly double the RPMs at 5200. Here is further data from tennisspeed.com:
Well, we have here Nadal, Djokovic and Roddick with the highest RPMs and ball speed. But why would one choose Nadal and Djokovic's forehand above Roddick's or Federer's? The answer would be control of sidespin.
The ability to hit sidespin, due to the conservative Western grip would explain the top 2 players in the world's stranglehold on the tour. Sure, they will lose a match here and there, and it's a temporary shocker when it happens. But in the course of time, who do we see in the finals. Both or one of the two.
It used to be that Sampras with his high RPM serve would dominate the tour, backed by his volleying skill. Now, it is the inside out forehand and the ability to control the rally.
Year after year, I see juniors drive the ball in search of the big strike forehand which elicit oohs and aahs from their parents and coaches. But I don't see a steady consistent kid who will eventually do what Djokovic and Nadal can. Hit a steady rock solid forehand which will get the ball in consistently and with creativity.