Simply a match where that day he was the much better player and should have won, and he lost. That certainly happens in tennis, and fairly often at that. And that’s a solid counter-example to show your metric doesn’t work as intended at all, since that’s at least 1 common case that makes the player looks good in your metric while it’s not good at all for the player. if you ever did some high-school-level maths, you know a simple counter-example is sufficient to prove something doesn’t work as intended, and this is a pretty big one.