Nando. Based on current form he should be able to handle Almagro reasonably comfortably (given also that Nando's never lost a set to him in two previous meetings).
Nando's serve tactics are intriguing. He served a 232 km/h (144 mp/h) bomb at one point in the match - the fastest serve of the tournament so far, and yet his average first serve-speed for the whole match was 179 km/h.
I figure he's saving energy.