Originally Posted by Rafter_forever
with the correction, Karlovic, for example, 2 out of 3 of his first serves are not returned by rival .. Am I right?
That's correct. To be precise, this is the percentage of first serves that are not neutralized to a rally. If you barely get it over the net for Karlovic to hit a sitter, this would count as an unreturnable.
Of course this is just an estimate but I think it's fairly accurate. Here is how it works:
67.7% of Karlovic's first serves win him the point directly
Of the remaining 32.3%, he wins 0.468 (as estimated from his rallying ability)
So in total he wins 67.7% + 32.3%*0.468 = 82.8% behind his first serve, which is the official stat by ATP
On the contrary, someone like Federer wins a lot behind his first serve but a big chunk of this is his rallying:
52.2% + 47.8%*0.547 = 78.4%