Yes, Delpo is the 5th best in the world behind Djokovic, Nadal, Murray, and Fed.
Ferrer doing great against the rest of the field means jack squat if he can't beat the top 4.
Exactly, you put it perfectly.
How can you be the fifth best player when the first four don't fear you in any way? Conversely, all of them know how good Del Potro is and that he can compete on even terms with them when peaking - Federer and Djokovic are even on record, on different occasions, saying that facing Del Potro is the same as facing another one of the top 4 from a mental point of view.
Unlike Tsonga and Berdych, Del Potro doesn't need to take any extra risks, when healthy for a prolonged period/in form, he can match any of the top 4 from the baseline and go toe to toe with them.
Well, Lenders, here's where your arguments begin. Your definition of a better player isn't the definition some other posters use. The answer is obviously Delpo. I would always pick him to beat a top guy or win a Slam ahead of Ferrer. And I agree, to me Delpo is a better player, but I totally understand people who say otherwise.
I can't. Is Ferrer fully fit/healthy more often than Del Potro? Unfortunately yes. Is he a better player? Absolutely not. Taking it as a given that the best four players are Djokovic, Nadal, Federer and Murray, there's not a single one of them who would not choose to face Ferrer over JMDP in a heartbeat if a big match were on the line.
Well, it can't be literally everyone because he would need some wins to have a ranking in the first place. But he could stay in the top 100 by 'vulturing' challengers, and meet the 'big 4' in R1.
As to the second part, let's say he's a 'Gulbis' type. He has no real motivation to play tennis, unless it's against the top 4, where he plays out of his mind. But he'll always lack the commitment to do any better. Is he the fifth best player in the world?
In that case, he'd be the fifth best player in the world when taking tennis seriously. Of course it's hard to get any sympathy while showing such lack of commitment. But yes, it'd not be outrageous to claim that such player at his best was the fifth best player in the world, although not consistently of course.