Murray always looks in control when the two face. Murray is just about as bad a match for Del Potro than anyone, considering he is an excellent mover who can slice defensively and offensively and has good attacking attributes in his serve and potentially his backhand. In other words, he can force Del Potro to make the error more than other players, as well as take time away from the poor movement of Del Potro.
On a perfect day, Murray would be lackadasical off the forehand and passive, whilst serving 50%, giving Del Potro the initiative. Realistically, Murray would force Del Potro to play out of his skin. Serving well, moving and anticipating well, timing the ball exceptionally well from different heights and maintaining concentration throughout. Nadal was below par, on a court he doesn't necessarily like, after having suffered a long injury lay-off. What Nadal did that day was serve poorly, as you'd expect, leave too many mid-court balls and produce more errors than you'd expect from him in normal circumstances. Nadal was void of confidence and Del Potro was the complete opposite.
You just cannot compare. The fact Murray knows he has Del Potro's number and Vice Versa doesn't hold well either.
I certainly would say Del Potro's win at the US Open was more impressive than Murray's considering he relied on an absolutely enormous choke from Cilic and ridiculously windy conditions and still being pushed by a poor wind player like Berdych and then a hindered Djokovic. That doesn't mean to say there was an element of luck in Del Potro's win, but less so and not a particularly alarming amount.
Murray is a bad match up for Del Potro, but if we are comparing peak vs peak, that is irrelevant. And still, despite being a bad match up there matches haven't been very one-sided.
In Rome 2008 JMDP was outside the top 50 and was a shadow of the player he would become and still the match was close untill he retired.
USO 2008 was after Delpo's breakthrough that was in the autumn of 2008 when he won 4 titles in a row (altgough he still wasn't at his peak). The match was very close 7-6, 7-6, 4-6, 7-5. JMDP served for the 1st set and was a break up in the 2nd set or the 4th one, can't remember which one.
Miami the same year was a pretty comfortable much for Murray.
In Madrid Del Potro won.
In Montreal it was very close untill DP fell apart in the 3rd. He almost wins that match though.
The WTF match wasn't easy but not so hard either.
It Murray's peak is really that superior, their matches should have been won much more comfortably by Murray considering he is a bad match up for JMDP. I think peak vs peak it's close, if they face each other at the USO Murray would be the favorite, but not by a big margin given how close the match they played there was.