Elsa, I'm not sure you were asking me or Slasher, but it was kind of a strange match for me. Federer appeared nowhere near as sharp as the previous 3 matches. Paire was very good at times, and not so good at others, especially when it counted, whereas Federer was almost the opposite. I would think many may not have been impressed with the overall level of play from both men. Paire was +7 on winners vs. unforced errors, while Federer was -7, which was much worse than he had shown on the previous nights.
Now, my take on some of the potential reasons for this play based on my experience.
Some very experienced and excellent players have the ability to purposely lower their level of play against opponents that have been playing well, to make the opponent play worse. This may have been what Federer was doing to Paire. Probably, only Federer would know.
Or, Federer has played only at night in Rome (which he does not love to do these days due to family), and last night played quite late (till just after midnight) vs. Janowicz. He couldn't have had much rest. Maybe he was rather flat or a bit tired or may have been trying to save himself as much as possible, knowing Rafa is lurking in the final. Again, only Federer would know.
Or, perhaps Paire was playing in a way that led Federer to not play at his best.
But it seemed when it really counted, Federer was able to lift his game to do what was needed, while Paire's got worse under real pressure.
A casual watcher may have thought Paire deserved the first set, at least up until the tiebreaker, where he was rather woeful.
In, the second set, Federer had more control after he broke Paire, but one couldn't be sure given his overall lower level of play.
I'm sure others (mostly haters) will state reasons like Federer was not playing well due to the Rafa Final factor, or that he is getting old and can't play more than 1 or 2 good matches consecutively, or that he is done, and should retire... but I don't give these much significance.
I'll be curious to how Federer and Nadal play tomorrow and how the weather will pan out. Forecast is for cooler weather (high of 20 C), rain in the morning, then clearing and somewhat breezy conditions (20 km/hr) at match time. Damper, cooler conditions might restrict the usual higher bounce during the day and favor Federer and allow him to play aggressively. But if it is not like that, and Nadal can produce his usual high bouncing topspin, I think it will make things tough on Federer.