Portugal take on tournament hosts Russian Federation in their next group-stage outing on Wednesday afternoon, before then facing outsiders New Zealand three days later.
Portugal had an early effort controversially disallowed by the video referees but they eventually took the lead after 34 minutes through Ricardo Quaresma after a mistake by Carlos Salcedo let Cristiano Ronaldo through on goal.
The game's first controversial moment came in the 20th minute when a free-kick by Ronaldo hit the wall and when the rebound came back to him, his left footed volley beat Guillermo Ochoa in the Mexico goal but struck the crossbar.
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Portugal coach Fernando Santos said it was not the performance he expected from his team.
As ever though, much of the focus will be on Ronaldo - Portugal's record scorer on 73 goals - and who will perhaps face even more scrutiny given the recent tax scandal that has arisen in Spain, and that has led to rumors he is seeking a move away from the Bernabeu. Portugal has won six of its last seven games. "We are calm and thinking about the next game".
Portugal's next game is on Wednesday at the Spartak Stadium against Russian Federation, while Mexico will face minnows New Zealand later on the same day at the Fisht Olympic Stadium.
Portugal and Mexico both made changes as the hour-mark approached in a bid to find that killer second goal, with Dos Santos and Andre Silva coming on and as the clock ticked, the game began to get stretched.