Arsenal and Bayern Munich head into the second leg in Germany with everything to fight for, having played out an intense 2-2 draw at the Emirates Stadium.
Leandro Trossard's sweet strike 15 minutes from full time was the all-important equaliser for the Gunners. They took the lead early on, with an outstanding left-footed curling finish from Bukayo Saka.
However, their lead was shortly-lived. Serge Gnabry found the back of the net after pouncing on an array of defensive errors from the Arsenal cohort, and Harry Kane gave Bayern the lead 14 minutes later from the spot.
Substitute Trossard did come to the rescue, slotting into the bottom-left corner after some excellent footwork from Gabriel Jesus inside the penalty area.
Here are four things we learnt from Tuesday's enthralling affair:
Arsenal's inexperience tested
Although the Gunners have been involved in some high-stakes, intense and demanding matches in the past two seasons, Mikel Arteta's side found themselves in unknown territory for the first time since 2008/09 - the UEFA Champions League quarter-final stage.
Aside from the former Chelsea duo Jorginho and Kai Havertz, the starting XI that the Spaniard opted for consisted of players in their debut Champions League campaign, with an average age of 25.2.
While the pressure of the fixture itself is heavy, their opposition in the form of the six-time winners Bayern Munich have an abundance of experience, fielding experienced professionals such as Joshua Kimmich and Leon Goretzka.
Indeed, the aforementioned players showed their experience, responding well to Arsenal taking the early advantage and staying calm.
A defensive mix-up between Gabriel, David Raya, Jakub Kiwior and arguably Declan Rice stepping forward a fraction too early allowed Der FCB to be clinical and thus gain momentum when the Gunners were in total control.
From there, the German giants managed to find open spaces and exploit Arsenal's nervous backline, with Leroy Sané splitting apart a sea of red and white shirts with a mazy run before being taken down by William Saliba inside the penalty area.
Mistakes may come back to haunt the Gunners, but playing knockout football gains valuable experience that they can use regardless of whether they attain a result in Munich and do progress. Playing a side such as the Bavarians provides a stern test tactically and quality-wise, which was nowhere near the level they faced in previous seasons in the Europa League.
Bayern's direct counter places spanner in Arteta's works
Undoubtedly, each team remaining in the Champions League has done their homework on the opposition, finding ways to derail what has been so successful for them in their domestic leagues.
Porto welcomed Arsenal with a lesson of the 'dark arts', time-wasting and calling soft fouls to keep the tempo slow throughout the game. It won them a 1-0 advantage to take to London, although the Gunners managed to battle valiantly through that in the return leg.
Bayern Munich were very different; they were composed in possession but able to strike a lethal counter-attack. With Sané and Gnabry on both flanks, paired with the rapid Jamal Musiala drifting from a more central position, the Arsenal defence struggled to consistently mark them like they had managed to against Manchester City and were often caught out of possession.
Although Arteta labelled in the post-match press conference that the Ben White opportunity one-on-one against Manuel Neuer was the 'moment' that could define the game, Thomas Tuchel may argue that it was White's crucial block that denied Der FCB from running away with the match.
With Sané darting through clear on goal, White and Martin Ødegaard were able to foil the German's opportunity, preventing a fatal third goal to bring back to the Allianz Arena.
The direct counter from Tuchel unsettled the Arsenal defence. As a result, it made the Gunners look like a nervous side - something that has not been seen so far in the calendar year.
Only at half time did Arteta intervene, opting to bring on Oleksandr Zinchenko in place of Kiwior. The substitution, combined with Sané leaving play early in the 66th minute, removed the threat of Bayern being able to exploit both flanks.
Gunners' patience is rewarded
Whereas Arsenal may have been reluctant to remain calm in previous seasons and become erratic when conceding two in quick succession, the maturity of the side seemed to prevail in the second half.
The Gunners moved the ball quickly but assuredly, with a few off-brand passes from Zinchenko the only anomaly as he was settling into the game.
With no luck breaking through a well-organised, experienced Bayern defensive unit, the game started to turn into a game of chess where pawns were just hopelessly being exchanged.
However, openings were available and with patience, Arsenal were able to find a reward. Jesus' intelligent run and fake shot drew Bayern towards him, and his lay-off to Trossard was inch-perfect, allowing the Belgian to slot cooly into the bottom corner.
Their consistent breakdown of the opposition's backlines, akin to the Manchester City philosophy, has become a key aspect to finding the back of the net, as defences can't deal with the relentlessness of the Gunners' midfield.
Therefore, it is no surprise that Arsenal have become much more patient this season, having scored 17 goals in the Premier League this season between minutes 75 and 90 (including injury time) - only bettered by an astonishing 27 from Liverpool.
Arsenal need to stop Kane's midfield presence at all costs
While Harry Kane may be feared for his goalscoring abilities, the 30-year-old arguably was more valuable in a midfield role during transition play.
The England international often dropped to the midfield line when Bayern launched a counter-attack, allowing the midfielders to combine with Kane, who could then showcase his spectacular range of passing by finding Gnabry, Musiala and Sané.
To deny the Bavarians from finding openings on the flanks, Arsenal will have to pay much closer attention to Kane when he drops in, ensuring that he is not able to enjoy the ball as much as he wants to.
If they do so successfully, the chance of keeping Bayern from scoring and progressing to the semi-final of the Champions League is very promising.
