It seems that with the advent of televised matches and attendant commentary, American coaches and parents have become obsessed with statistics. Talk about unforced errors and first serve percentages has turned the audience into a generation of record-keepers. Perhaps it's cultural - drawn from the fans obsession with statistics in "sports" such as baseball where if it weren't for a religious fascination with numbers the commentators wouldn't have anything interesting to discuss.
When it comes to tennis, however, I often see the focus on first serve percentages as an obstacle in development. Listen, after 30-40 years of getting hammered with first serve percentage figures or the importance of not making any unforced errors, you'd think that we, as a nation, would be completely dominating the international tennis scene. This is obviously far from being the case.
So how is the focus on first serve percentages detrimental you ask? Well, in my opinion, it is not so much in what is said, it's how the message is perceived by the student that causes a problem. In this case, if the coach keeps dwelling on the first serve percentage, the player usually holds back on the serve and practices only to "just make" the ball in service box. This usually means slowing the ball down and hitting quasi-second serves.
Although I'm a HUGE fan of second serves, unless you're using the serve simply to 'kickstart' the point (pun intended), if you want to put some pressure on the opponent throughout you're going to go after some first serves. In theory, I think you should go after your first serve most of the time though sometimes you put slightly more spin on the ball in order to create some angles and Often times, however, players are simply incapable of hitting big first serves because all they've done in practice is hit their shot with too much emphasis on making the ball. In other words, they have practiced hitting it slow. In a recent lesson, one student would hit 1 big first serve and, if he missed it, went back to hitting 10 slow second serves in order to build confidence. So, in 30 minutes of serving, he's hit 15 big first serves and 150 second serves. Then he's wondering why his opponents - after they get a good read on his "first" serve - start hammering him with their returns.
The point is that if you practice slow, you're going to hit slow. It's like a sprinter who, while training for the 100m sprint, is running long-distance races in practice. The stride, just like the serve, is simply different. No coach in their right mind would tell Usain Bolt to run 1 mile circuits in training while focusing on the short-course sprint. If you want to run fast - practice fast.
Same with the serves, if you want to hit "hard" during during match, practice hitting hard while training. Train your fast-twitch muscles to be explosive. There's entirely WAY TOO MUCH emphasis in instructional videos, reading materials and lessons on particular components of the stroke and not enough focus on actually getting the "fire" (or anger?) in the player's belly to GO AFTER THE SERVE.
Don't misunderstand what I'm saying, however. DO focus on the technique!!! But focus on the technique WHILE you're hitting hard first serves! Otherwise, you're like a race-car being fine-tuned while driving circles in the parking lot and don't know if you're capable of hitting laps at 230mph. Often times, a slight variation in degree (string bed contact with the ball) will send the ball 10-20 ft off course ( due to speed of racket; depending on mph). This means that the player needs to develop a better "feel" for the ball at that speed - not to drop back and hit an entirely different stroke. So practice hard and make small adjustments in the process. If today you can only hit 10 good, hard serves - great. Tomorrow go for 12, the next day 16 and so on. However, as a high level player you need to develop the stamina AND the explosiveness to hit hard first serves for 5-6 sets/day (i.e., in a tournament). You're not going to do that practicing only soft serves (regardless of how many you actually manage to make in the box).