Lol @ Ferrer being "too good". Kei beat him in a major as a youth!
Anyway, this is one of the more worthy "WWW"'s (most have been arbitrary). I'm gonna say Kei does it in 4: 6-4 7-5 3-6 6-4
And not to make excuses, but Nishikori actually was having his ankle taped during an MTO while he was up like a set and a break in yesterday's match, so he could have legitimate issues going forward.
It's funny that medical issues are a legitimate reason to dismiss any of Ferrer's victories, but you seem to overlook the fact that when Kei beat David "as a youth", David was having some major personal issues and possibly a big depression, and that it was the beginning of a long slump for him.
The 2012 Olympic encounter would be a better reference, but I don't agree that Ferrer was "in form", it was actually the one period in 2012 when he was not that great. He had a great year, but with a small slump between Wimbledon and the USO (tiredness, illness, a bit of both).
Plus, it was on grass, which is still Ferrer's worst surface.
However, don't mistake me, I do believe Nishikori is a real threat. He seems to be solid, while Ferrer's true form is a bit of a mystery to me so far. So anything could happen really. I'm gonna say Ferrer in 5, but that's more wishful thinking than an actual prediction.