Just watched de la Hoya -- Mayweather 2007. Of course, Mayweather had a slight age advantage then while Alvarez has a major age advantage tonight. Still, the 2007 fight might be informative.
The announcers and the crowd basically gave the first 4 rounds to de la Hoya, in part, because for the first time ever Mayweather seemed to be being forced to fight backing up with de la Hoya was being credited for a bunch flurries that weren't really landing but were looking better than they were. By the middle rounds the announcers were calling it more even and were talking about Mayweather' s great defense. By the last 3 rounds it had become totally obvious that de la Hoya had taken much more punishment in the earlier rounds than Mayweather and was clearly losing (ie Mayweather had used the first 4 rounds to figure him out and then made him pay for it).
Another very interesting thing was that through the whole fight the announcers did comment on the fact that from the start between rounds Mayweather was barely breathing between rounds while de la Hoya was usually hyperventilating. Given the age difference tonight in Alvarez's favor tonight it will be very informative if something like this happens again tonight.
At any rate, keep in mind that for awhile early in the fight Mayweather's opponents often seem to be doing better than they really are, and even though they are not really landing, as Mayweather goes about the process of figuring out the best way to attack them.