It was nearly a dreadful start for the visitors when Gerard Pique looked to be the last man as he brought down Ronaldo on the edge of the box, but the Spaniard was issued with just a yellow card.
From the resulting free kick, the Madrid skipper forced Jose Pinto into a stunning save to his right.
The hosts came close again just a few minutes later, when Michael Essien's deflected cross was palmed over the crossbar by Pinto.
The away side created the first great chance of the match in the 14th minute when Andres Iniesta chipped a pass through for Jordi Alba from the edge of the penalty area, but the full-back could only slice his effort across the face of goal.
Barca came a whisker away from taking the lead just 10 minutes later as Xavi tried his luck with a free kick from the edge of the box, but his powerful effort thundered off the top of the crossbar and away to safety.
The away side had a chance cleared off the line two minutes later, when Varane was able to hack away Xavi's shot after Ricardo Carvalho had carelessly given the ball away.
The home side had a good chance to break the deadlock with just seven minutes of the first half remaining when Jose Callejon burst down the right before firing a low cross into the box, but Pique was on hand to thump the danger clear before Ronaldo could reach it.
The hosts had the first chance of the second half when Karim Benzema danced past two challenges before firing an effort just high of the top corner.
However, it was Barca who took the lead just five minutes after the break when Fabregas collected a loose ball from Mesut Ozil before spinning in the box and smashing his effort into the bottom corner.
The hosts had a glorious chance to equalise with 29 minutes left to play, but Ronaldo somehow managed to head wide from a few yards out after Essien had floated in the cross.
The Catalan outfit nearly doubled their lead 10 minutes later, but after being picked out by Pedro, Fabregas could only manage to thump his effort against the crossbar from inside the box.
Barca had another close chance two minutes later, when Lionel Messi played in Pedro who drove forward into the Madrid box, but his delicate chip rolled agonisingly wide of the far post.
However, the home side equalised with just nine minutes left to play when Varane thumped a header past Pinto from a Ronaldo free kick.
The second-leg takes place in late February.