Check out these excerpts from the USTA FAQ:
Q: What is an NTRP Rating and what are the different categories?
A: An NTRP Rating is a numerical indicator of tennis-playing ability, from 1.5 (beginner) through 7.0 (touring pro), which aligns with a set of general characteristics that break down the skills and abilities of each level, in 0.5 increments. Ratings are generated by play in USTA Adult Divisions of 18 & Over, 40 & Over, 55 & Over, 65 & Over, Mixed 18 & Over, Mixed 40 & Over, Mixed 55 & Over, and Combo 18 & Over.
A: A player’s pro says their rating is higher than what the computer says. Can a player’s rating be adjusted based on what their pro is telling them, or can someone hit with them to see if their rating can be adjusted?
Q: No. Teaching professionals have a general sense of how someone may compete at various NTRP levels. This enables them to help connect new players with other players who may have a compatible style of play. However, they are not trained by the USTA to accurately assess a player’s exact NTRP rating. Ratings are based on matches, not how well someone can hit their strokes or play points in a lesson. In addition, a formal visual verification process is no longer recognized by the USTA.
Q: Is there a rating professional, or someone else, who can observe a player to support a claim that they should not be disqualified?
A: No. For consistency and fairness, all players are judged by the dynamic calculations [computer algorithm based on match results], not human beings.
Find answers to your frequently-asked questions about USTA League and NTRP.
www.usta.com
Ratings are generated by results from official match play, period.
How else do you measure how good someone is at tennis, if not by their ability to win matches?
They even got rid of the old visual ratings, where they would have a guy watch someone play and then give them a rating based on their strokes and stuff. That's closer to what you are thinking. The reality is, how good someone looks doesn't necessarily correlate with winning or losing. And winning or losing is the only true, objective measure of tennis ability.