Cristiano Ronaldo was back to his best.
A Pepe header was the difference in a dour game between the two last time out, and with a potential semi-final against either Atletico Madrid or Athletic Bilbao at stake, both teams were looking to improve on their performances that night.
Let's take a look at six things we learned from the game...
Espanyol's lack of shape allowed Luka Modric and others to exploit the space with ease.
To some extent, the home side was the author of its own downfall.
Javier Aguirre had set his team up in an attacking 4-2-3-1 formation, but it was seen very infrequently over the 90 minutes.
During the first period, two tight banks of four were often 20 yards or more away from the front two, meaning any out ball rarely reached its target.
Madrid's defence picked off any wayward Espanyol passing, and this profligacy was the chief reason why we saw the away team rattle off seven goal-bound shots in the first half.
Espanyol improved over the course of the second 45, but their setup still left a lot to be desired. Cristiano Ronaldo's headed chance midway though the half came directly as a result of the home side committing far too many men forward.
As Angel Di Maria won the ball deep into his own half, there were five Espanyol players in an attacking line across the field—three in midfield, leaving just two at the back.
This is scandalous at this level of football, and these two instances were by no means isolated incidents.
Gareth Bale carried on his good form.
Another assist, a venomous free-kick, which deserved a goal, and plenty of decent link-up play all suggest there is a lot more to come from the Welshman.
His contribution tonight was, perhaps, more understated than normal given that he didn't add his name to the scoresheet, but his all-around play certainly helped put last season's runners-up in the box seat in this tie.
An 83 percent pass accuracy rating and winning all of his aerial battles led to a performance score of 7.4, per whoscored.com, which compared very favourably to every other player on show tonight.
Kiko Casilla was beaten by Karim Benzema's superb header.
Games are often decided by such fine margins, and it highlights the need to take your chances when they come your way.
Karim Benzema had to react quickly but still managed to dispatch his header with power past Kiko Casilla.
Compare that to far more presentable chances from Espanyol's Pizzi and Jhon Cordoba. Put those away, and the tie is still wide open.
In both cases, it was easier to score, and TV replays showed exactly what manager Javier Aguirre thought of the misses.
Iker Casillas is too good to be considered a reserve goalkeeper.
Spain's record appearance holder and one of Real's longest servants: Mr. Dependable.
How is it that Iker Casillas has been relegated to a Madrid also-ran and is now only considered good enough for a cup run or a friendly?
It was in last year's Copa Del Rey quarter-finals when Casillas picked up the injury that eventually led to the emergency loan of Diego Lopez, and Casillas showed again tonight, particularly when going one-on-one against Jhon Cordoba, that he has lost none of his ability.
So what exactly does he need to do to force his way into Carlo Ancelotti's thoughts and regain his rightful place as the No. 1?
With no discernible weakness to his game, lack of match practice and the form of Barcelona's Victor Valdes must surely be harming his World Cup chances, despite any assurances from Vicente Del Bosque.
Luka Modric and his team-mates hold a definite advantage in the tie.
Javier Aguirre's men will have to throw caution to the wind at a ground where they traditionally struggle.
Indeed, the Catalans haven't tasted victory at the Santiago Bernabeu in any competition for over a decade.
With Carlo Ancelotti's men recently building up a decent head of steam, the second leg should be a foregone conclusion, although the second-half performance from the home side gives Aguirre a glimmer of hope.
Javier Aguirre's men have got a mountain to climb if they went to progress from this tie.
Espanyol's patchy form has continued throughout the season, and tonight was yet another example of the Periquitos flattering to deceive.
In truth, an aggressive Madrid side didn't allow them too much room to execute their passing game for large parts of the 90 minutes.
It's a common theme that should be concerning Javier Aguirre, given his team's flirtation with the relegation places. Just six points separate Espanyol from the bottom third.
A battle to avoid the drop is probably not what the manager had in mind for mid-January.