Fitness vs Form?

I seem to encounter this dilemma quite regularly. Late on in the match regardless of score I will have players that are on form or at least playing better than the rest of the team but are dropping quite a bit in fitness. Maybe through working harder which leads to fatigue. Then I have to decide whether to bring off that player(s) who are playing the best but knackered! My question to the community is do you sub on form players for fitter guys or leave the best players on even though they are deteriorating? I guess it depends somewhat on if you're winning or losing but in general.

In general I take off players that are below 70% as I feel that their performance drops off a bit. If I'm losing, I might keep them on and take off a rubbish but fit player instead.

If they are key players, i will generally leave them on, unless they are absolutely exhausted (below 70%). But usually i will sub players as soon as they go below 80% or even earlier if i am winning.

Surely it depends mostly on the score.

If I'm chasing the game and my attackers are looking highly motivated with good ratings, I won't sub them; I'd go for someone who's not having such a good game, or make a tactical substitution (e.g. take off a defensive player for an attacking one). Generally, a player with good motivation etc. performs better than one with good condition.

If it's 4-0 for my team, of course I'll take off my star players to keep them fit for the next match.