Ronaldo teed up Ricardo Quaresma to fire Portugal ahead in the first half in Kazan, but the goal was cancelled out before the break by Mexico striker Javier Hernandez's bullet header.
Cedric Soares restored Portugal's lead on 86 minutes, but Hector Moreno's stoppage-time header salvaged a point for Mexico in their Group A opener.
"We could have killed off the match but it didn't happen. The draw is a fair result without doubt," said Portugal coach Fernando Santos, whose side wore black armbands in honour of the victims of a deadly forest fire back home.
"I would like to say something on behalf of my players. People are very sad in Portugal and the players are thinking of all those who are suffering from what's happened."
Mexico coach Juan Carlos Osorio said his team deserved their point but admitted there was work to be done going forward.