The very well developed essential elements of a good service are here mainly a well-controlled toss, excellent eye control of the ball, a good build-up of a tension in the body and a sound understanding of the targeted pronation channeling energy against the target in the first part of the follow through. Such a solid Service 3.0 is an excellent base for further growth and shows that good developmental technical work on service was done with Kayla in the past.
Photos below show Casper's service during 2015 Orange Bowl. Even thought that his service was not that dominant back then, the most important basic characteristics of the Service 3.0 were already there at that time and provided a solid base for further development/improvement. The most important among them is the proper understanding of the "follow through 1" with a well-executed (well-pronounced) targeted pronation.
Gael belongs to the best servers on the pro tour and his service mostly perfectly fulfills the highest Service 3.0 criteria of body energy dominance of the stroke. A typical sign of this is Gael's excellent targeted pronation in the "follow through 1". He appears to have discovered the essential aspects of the TENNIS 3.0 CODE for the service.
This former top 10 player in both singles and doubles as well as twice Davis Cup Champion with the team of the Czech Republic belongs to the best servers on the Tour from the technical point of view. Despite his just average body size of 185cm (6'1''), he has a quite high career ace probability of 51% per game played.
Regarding service, forehand and backhand, Marin Cilic can actually serve as a certain standard prototype for the TENNIS 3.0 style of stroke production, where body energy dominates the stroke dynamics as well as racket position in space and where elbow reaches longest possible distance from the center of gravity during and after the impact.
While Serena's service is being considered as the best in the game, service of her sister Venus is not much behind..., besides the technique also her tall body of 185 cm (6'1'') helps. The situation that Venus tends (mainly under stress) to tilt her head to the left side before the impact might be one of the reasons for her rather higher double fault rate than the one of Serena...
Roger's service has more pronounced pronation and he spends a higher percentage of the service energy on the dominant (=right) side of the body than Andy. Over his entire career on the tour (data until mid-January 2017), Federer has the 1st service percentage of 62% (Murray 58%, Djokovic 65%, Nadal 69%) on which he won 77% of the points (Murray 75%, Djokovic 73%, Nadal 72%)...
Already in the late 90's, Mirjana belonged to the most dominant serving female players on the WTA Tour as she was one of the first of them who really understood the efficient use of the pronation. On the photos from 1998 shown below is well visible how fast she was able to pronate with the integration of the body energy already in the age of 16 years
As a tall player of 198 cm (6'6''), Delpo has quite some power potential, which he is using in all of his strokes. His body parameters and solid technique make also his service to a dangerous weapon, despite partly missing one distinct element common to the very best servers of the history. His career service statistics (until the end of 2016) are quite impressive - in the total of 5563 service games - 3272 aces (against 1100 double faults), 64% of the 1st services in and 74% winning percentage on the 1st service points.
Denis, who is being trained by his mother Klaudiya Istomina, possess quite solid and efficient stroke technique and his service fulfills all the requirements of the Service 3.0, which stands for service played with optimal body energy integration. Some details of his service can be seen on photos from the 2016 US.Open shown below: