They're rivals in the sense they contend with each other but Almagro really has no cards on Rafa on a day to day basis unless he plays lights out and makes his balls in for long enough. Last game saw some good stuff from Almagro but too wild otherwise.
To assist, some Merriam Webster definitions:
Rivalry = competition for the same objective or for superiority in the same field.
1) a person or thing that tries to defeat or be more successful than another
2) something or someone that is as good or almost as good as another person or thing
So based on this, Almagro is a rival in the sense that he plays and tries to win (even if he loses more often) but doesn't fit the second where he just isn't as good as Nadal. So let's just say that the Almagro-Nadal matchup is a rivalry such that Almagro is a rival to Nadal but simply cannot rival Nadal's game.