Ronaldo will be looking to continue his fine scoring run, having bagged 10 goals in his last five league matches.
Paco Alcacer took the place of suspended striker Luis Suarez in the starting line-up and scored a first goal in three months to put Barca ahead in the eighth minute at the Nou Camp, side-footing home a pass from Jordi Alba.
The lead was to reclaimed two minutes from the break after Ronaldo unleashed a powerful shot that struck the underside of the bar before hitting the back of the net.
"Of course, Champions League games are decided by the small details".
"He comes from another galaxy, it's true", the head coach told Real Madrid TV.
Girona were fortunate not to fall two behind midway through the half as Vazquez's finish from Kroos' delightfully disguised ball was incorrectly disallowed. "There weren't many chances in the second half, for us or them".
The ties will be played on April 3 and April 11 with a derby against Atletico in between.
Speaking at his media conference, Zidane added: "It is very important for us to transmit important and positive energy to the rest of the group when it is better for the group and for everyone".
Cristiano Ronaldo scored four goals in a nine-goal thriller as Real Madrid demolished Girona by 6-3 at the Santiago Bernabeu.
Lucas Vasquez and substitute Gareth Bale also found the net for Real, who looked sensational going forward but remain vulnerable in defence.
According to Marca, Ronaldo made a bet with his Real teammates four months ago that he would beat Messi to the Pichichi trophy, which is awarded to La Liga's top goalscorer every season.
"On a personal level he has 22 goals, three behind Messi, so I hope [that goes well] for him, as when he is good, the team is good".
"Cristiano's ambition is tremendous".
"But I also want to highlight his colleagues, who help him a lot".
Girona scored for from another set piece for the third time in the match as Juanpe rose highest to score giving Keylor Navas no chance.