The game was tied 1-1 in the 44th minute when Ronaldo restored Portugal's lead.
Russian Federation selected 11 host cities to be the venues for the matches of the 2018 World Cup and they are Moscow, St. Petersburg, Sochi, Kazan, Saransk, Kaliningrad, Volgograd, Rostov-on-Don, Nizhny Novgorod, Yekaterinburg and Samara.
Given all the turmoil during a week which saw Hierro step in after former national coach Julen Lopetegui was sacked for agreeing to take over at Real Madrid after the tournament, the new boss told Spanish TV station Telecinco that he was happy the team had stepped up and showed their personality.
Diego Costa had a great day for Spain as well, putting two goals (both equalizers) of his own in, but people were understandably enraptured with Ronaldo's performance. Nacho Fernandez then put the Spanish ahead with a one-timer from outside the area in the Group B match.
With his hat-trick, Ronaldo also became the first player to score in eight consecutive major tournaments.
Each player has won the FIFA Player of the Year Award five times: Messi from 2009-12 and in 2015, and Ronaldo in 2008, 2013-14 and 2016-17.
"I am very happy for the goals but what happened at the end was a pity", Costa said.
Ronaldo downplayed his sixth global hat trick. More importantly, it evened the score at 3-3.
But it was Ronaldo's second goal, a relatively tame shot from outside the box, that's seen de Gea fall under a wave of criticism. Lopetegui would have certainly had different plans for Spain's campaign.
"It wasn't an easy situation".
De Gea said he felt "calm" in the wake of his mistake , insisting "only those of us who put on our gloves and go out in the field know how hard [it is]" and quickly turning his attention to winning the next game.
Pique fouled Ronaldo to award Portugal the set piece and the Barcelona defender says the Real Madrid star is known for diving. Goncalo Guedes controlled a long ball and squared it for his captain, who had time and space to place a low, powerful shot that inexplicably slid from De Gea's grasp and rolled into the goal.