Most vulnerable to lose is Andy Murray. Nadal is rolling while Federer seems to find the right shot at the right moment but as to Andy, he is shaky on clay....

Nadal is most likely to get upset tomorrow

Federer will win

federer

Ferrer should beat Murray on clay but sometimes when playing in front of his home crowd he gets a bit too over emotional when he misses a few shots he should make, that could unsettle him and let Murray off the hook. So Murray in 3 sets.

Gulbis could beat Federer unless Federer plays in demon mode it depends whether Federer really wants to make a statement in the masters like he did last year in Cincinnati (where he beat both Murray and Djokovic in straight sets), there is a chance that Federer could wobble like he did last time as he hasn't won more than two matches in a row since the AO and with a talented player like Gulbis he won't be able to get away with lapses in concentration. Gulbis in 3 sets.

Monfils has zero chance of beating Nadal, he can never get his shot selection right on the key points, I was watching him play Garcia-Lopez and he should of won that match easily but he played some bizarre shots on the key points, Nadal will deal with him easily. Nadal in straight sets.

As for the fourth match, I hope Melzer wins, both he and Almagro will just bow down to Nadal but at least Melzer isn't spanish, but I think Melzer is ranked higher so it isn't really an upset.

Ferrer to beat Murray. Federer and Nadal will make it through to the semi-finals.

Predicted scorelines
Ferrer defeats Murray 6-2 7-5
Federer defeats Gulbis 6-3 7-6
Nadal defeats Monflis 6-4 6-4