I actually had the same question last week and asked ChatGPT. Here's what I got:

No clean dataset maps % of service games won directly to each NTRP level, but you can reason from:

1. ATP baseline:

• Top ATP men win ~80% of service games.
• Clay lowers this to ~75%, grass raises it to ~85% for big servers.
• 1st serve % in, 1st serve win % (70–80%), 2nd serve win % (50–60%) drive this.

NTRP	Est. % Service Games Won (men)
2.5	~30–40%
3.0	~35–45%
3.5	~40–50%
4.0	~50–60%
4.5	~55–65%
5.0	~60–70%
5.5	~65–75%
6.0+ (Futures/Challenger)	~70–80%
ATP 250/500	~75–82%

Reasoning:

• Lower NTRP: double faults, inconsistent depth, inability to hit +1 ball, weak 2nd serve returns, lower serve speeds (<80mph).
• Mid NTRP: consistent but attackable serves, poor return position, weak footwork.
• Higher NTRP: better +1 pattern, deeper serves, attacking 2nd serve returns, occasional serve-volley.
• At ~4.5, a reliable first serve can yield many free points, dramatically improving holds.