It was a night of tension and endlessly pouring rain in Valdebebas, as Real Madrid and Chelsea shared a scoreline but left the advantage firmly with the men in blue.
Thomas Tuchel's side will now hold the real belief that they can make the Champions League final in Istanbul, after Christian Pulisic's clever strike gave the west London club an all important away goal.
Madrid will return to Stamford Bridge with it all to do, yet Tuesday night's encounter gave an insight into the changes that could be expected in the decisive second leg and, perhaps, where the match could be won or lost....
Madrid need Ramos
Despite his aging years Madrid still rely massively on Ramos, this much is certain.
His power, presence, confidence and experience are vital for the way things operate in the back line.
In his absence, Zinedine Zidane simply tried to shut up shop. Madrid fielded a heavily defensive formation, incorporating a rigid central back three with Marcelo and Dani Carvajal flanking as wing backs.
It's fair to say this did not work. Chelsea broke the deadlock easily enough, and could have had a nailed on second had Timo Werner not missed from point-blank range.
Madrid looked unbalanced as a result, and Marcelo turned in one of his most underwhelming performances on the big stage for many years.
Ramos is desperately needed, and his return will not only breath hope back into the Madrid side but also allow Zidane to revert to a traditional back four and give his side more expansive options to go out in search of the goal they crucially need at the Bridge.
Mount can cut it at the top
For some reason questions are still being asked about Mason Mount, despite him being one of the top performing stars in the Premier League this season.
It did not take Tuchel long to find out, following his arrival, that Mount was so much more than simply former boss Frank Lampard's favourite - as many have previously suggested.
Mount is in the Chelsea team because he deserves to be, and against Madrid he proved that even among elite European company he does not look an inch out of place.
The England youngster looked fearsome in possession against the Spaniards, causing myriad problems on the left flank and had Madrid throwing extra bodies behind the ball when cutting inside.
His role will be crucial in the return fixture, when more space is set to be afforded to the Blues in the final third as Madrid press up higher in search of the goal they need.
Werner's role needs to change
It's been a rotten season for the Germany international, and so it was no surprise to see his miss from point-blank range dominate the headlines following the match.
In fairness to Werner he only had a split-second to get his shot off and direct the ball into the bottom corner from six yards, but instead fired tamely straight at the keeper.
It was the sort of chance in exactly the sort of high stakes game that Werner had been bought for.
The German was supposed to flourish on occasions such as these, and build upon his sizzling Leipzig form to propel Chelsea into football's biggest finals.
Now, however, his confidence has likely taken the most extreme of hits. Werner worked tirelessly after missing his chance to score, but did not look like an individual threat for the remainder of his time on the field.
Manager Tuchel now has a big decision to make.
Given that Chelsea have already bagged the crucial away goal, the German boss may likely bench his compatriot in favour of the likes of Olivier Giroud.
It's important to note that Werner played a key role in opening up space for Chelsea in sensitive areas across the back line, using his electrifying pace to keep Madrid defenders constantly occupied.
Tapping into this late on in the second leg, by introducing Werner as a sub, could work wonders for the Blues.
Should they be successful in pegging Madrid beg until the latter stages of the return fixture, Werner's introduction could have the potential to completely break down Madrid's attempts to pile all men forward - or run the risk of conceding a game-killing blow on a rapid counter.
It may have been a miserable evening for Werner, but he still has a big role to play in this fixture.
Shutdown Benzema at the Bridge
After hitting the post in the first half, Benzema wasted little time before showing another flash of magic and firing his side level.
The control to head the ball down and produce a clean volley on the pivot was textbook Benzema.
He kept his head as all around him lost theirs, before sweeping the ball through the rain-soaked air and into the top of the net.
Despite now being in the twilight of his career, Benzema showed he is very much the difference-maker for Madrid.
The ever observant Tuchel will have noted this, and will be acutely aware that the Frenchman is the main risk factor when the Spanish giants touch down in London.
To lock down Benzema will be to control Chelsea's destiny in the second leg.
Everything that was promising went through the central striker in Valdebebas, with Benzema using his strength and close control to hold the ball up for the likes of Vinicius Jnr playing alongside him.
Cutting out Benzema would enable Chelsea to nullify Madrid's focal point, and so initiate their own attacks out from the back.
It's easier said than done, but may well be the recipe for success.
It's not Hazard who can hurt his former club, it's Courtois
Much was made about Eden Hazard returning in time to face his former club, which saw the reunion of Thibaut Courtois' with his previous employers overlooked.
The Belgian managed to slip under the radar, that is until he pulled off a sublime reflex save to deny Werner from mere yards away.
Indeed, Courtois was helped by the German's complete fluffing of his lines, but he still needed to be in the right place at the right time.
Courtois left Stamford Bridge under a dark cloud and would most likely have been in for the frostiest of receptions upon his return, were fans permitted inside the stadium.
But they're not, and the tall stopper will have a peaceful hush in which to channel his thoughts and focus only on keeping out his ex-team mates.
On his day Courtois has proven to be among the finest goalkeepers on the planet.
In the previous round he was exceptional against Liverpool, with the pick of his highlight reel coming in the form of a sublime arching save to tip James Milner's fizzing effort over the bar.
Should Madrid get the goal they need at Stamford Bridge, all eyes will be on Courtois to see them through to the final and break Chelsea hearts.
Zidane must unlock Kroos and Modric... or a Tuchel defensive masterclass will book final place
After being so influential against Jurgen Klopp's side in the last round, where were Toni Kroos and Luka Modric?
The old guard simply saw the game pass them by and were all but lost among the rain on Tuesday night, as Chelsea's clever tactics simply unfolded around them in the heart of the field.
A defence-heavy Madrid set-up ended up soaking up much of possession at the back, enabling Chelsea to apply a high press and force a long ball clearance.
The result? Kroos and Modric found themselves cut out of play time and time again.
Zidane needs to tackle this, or he may as well resign himself to a thoroughly uncomfortable evening at Stamford Bridge.
As aforementioned, the return of Sergio Ramos will almost certainly shake up Madrid's formation, adding vital balance for the midfield to bend the game to their will.
Chelsea will counter this, however, so don't count against Tuchel devising a rather specific game-plan to take Kroos and Modric out of the equation once more.
If Chelsea can pull this off, particularly in a match where Madrid desperately need their creators to fashion goalscoring chances, plane tickets to Turkey will need to be booked.