This is really nerve-wracking! On paper, it should be Murray, but I'm notorious for making the wrong guess - so I'll go for Del Potto (to be on the safe side) but with fingers firmly crossed on the Q.T. .
It seemed to me that in DelPo's match he was getting less support than Nadal -- due to a Brazilian-Argentinian rivalry perhaps?
Anyway I suspect Murray will be able to cope with a hostile or mixed crowd. Whether he's able to cope with DelPo's vicious forehand is less certain, but if anyone can do it he can. Has been great to see Del Potro back to something resembling his best, though.