It's been two months since we last watched Champions League action, with the group stages coming to a close in early December.

But fear not, the 2020-21 competition is set to whir back into life on Tuesday as the knockout phase gets under way with the first batch of round-of-16 ties and the latest official match ball, which will be used all the way through until the final, has also been revealed.

20 years of history.
Introducing the new official @ChampionsLeague match ball, celebrating 20 years of the iconic starball design.
Available tomorrow from https://t.co/FblJVIGgpI#UCL pic.twitter.com/6iRp0x7Y7f

— adidas Football (@adidasfootball) February 15, 2021

The Adidas Finale Istanbul 21 is a nod to the iconic Adidas "starball" design, which is celebrating its 20th anniversary this year having first been introduced in 2001, and incorporates panel designs taken from every single one of the 20 previous Champions League final balls, with the thin red banners marking the relevant year and host city for each of them.

Now that the Champions League is firmly back on the agenda and the new ball is ready to roll, here's a refresher on some of the more notable things to have happened in the competition so far this season.

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Ronaldo overcame Messi

Two years (947 days to be exact) since their last encounter, twin modern day goliaths Messi and Ronaldo came head-to-head on the same pitch in the sixth and final Champions League matchweek before the winter hiatus.

Barca had a three-point cushion over Juve at the top of Group G heading into the match, but that soon vanished as Ronaldo and his cohorts emerged victorious. It was the 36th time Ronaldo and Messi had met on the pitch, and the former came out on top, scoring twice while his great rival struggled to gain any kind of foothold in the game.

Juve prevailed as 3-0 winners on the night, ensuring that they ended the campaign as group winners, having risen above their foes to seal top spot by the narrowest of margins.

- Ronaldo and Messi's most memorable clashes

Rashford thwarted PSG (again)

Marcus Rashford produced yet more late heroics as Manchester United defeated PSG on their own soil for the second year running.

There was controversy in the previous season as a stoppage-time penalty (given by VAR for handball against Presnel Kimpembe) from Rashford secured a dramatic 3-1 win win over the Parisians to gift United progress in the round of 16 on away goals.

Skip forward to October 2020 and Rashford once again left it extremely late (the 87th minute) as Ole Gunnar Solskjaer's side kicked off their Group H campaign with a 2-1 win at the Parc des Princes, though things did not go so well in the return game (see below).

Bayern Munich rumbled on

The Champions League holders look well poised to defend their crown, having gone unbeaten throughout the 2020-21 group stage.

The Bavarians are unbeaten in the competition since 2018-19, when they fell to a 3-1 defeat against Liverpool in the second leg of the round of 16, and since then they've played 16 games and won 15, with a 1-1 draw against Atletico Madrid at the Metropolitano Stadium in December the only (relatively minor) blemish on their otherwise flawless copybook.

Indeed, Hansi Flick's newly crowned Sextuple winners are in imperious form once again, having scored 18 goals in six group-stage outings thus far -- considerably more than any other team.

Haaland kept breaking records

After making a prodigious start to his Champions League career, Erling Haaland has continued his incredible form into his second season.

The Borussia Dortmund striker shattered more records by becoming the fastest player in history to reach both 10 and 15 Champions League goals.

It took Haaland seven games to take his tally to 10, and then just 12 games to score 15 goals -- a record previously jointly held by Ruud van Nistelrooy and Roberto Soldado, both of whom took 19 games to reach the 15-goal mark.

The 20-year-old Norwegian reached his latest milestone by scoring twice in Dortmund's 3-0 win over Club Brugge in late November, the second of which also happened to be the German club's 300th goal in the competition.

As things stand, Haaland is joint-top scorer in the Champions League this season, joining Alvaro Morata, Neymar and Marcus Rashford on six goals apiece -- and that's with injury ruling him out of Dortmund's last two group stage ties.

Real Madrid rode their luck

With consistency already an issue, Real Madrid got their 2020-21 group campaign off to a wretched start by losing 3-2 against Shakhtar Donetsk at the Bernabeu.

Los Blancos' hazy aspirations of winning a 14th Champions League title looked to be in tatters as they then lost the return fixture against Shakhtar, stumbling to a 2-0 loss in Ukraine in early December (their third defeat in five games in all competitions), leaving the Spaniards third in Group B with one game left to play.

Fortunately, Real rallied to beat Borussia Monchengladbach on the final matchday, with results elsewhere meaning that Zinedine Zidane's beleaguered side leapt up several rungs to somehow finish as group winners.

They came good when it mattered and squeezed through to the knockouts, but performances haven't exactly been stellar in the intervening months, and a humiliating Copa del Rey defeat against third-tier side Alcoyano a few weeks ago serving only to pile yet more pressure on Zidane's shoulders.

With seven important games still between Real and the final in Istanbul, how much luck do they have left in reserve?

Managers made some moves

Zidane is holding at Real Madrid on for the time being, but several Champions League clubs have relieved their head coaches of their responsibilities since the winter break began.

Despite finishing top of their group, Chelsea pulled the plug on Frank Lampard's 18-month stint in charge in January, replacing the club legend with Thomas Tuchel.

Similarly, Tuchel had been responsible for steering PSG to the top of Group H, but was fired in January to make way for former Tottenham boss Mauricio Pochettino to step into the breach.

Pochettino will now be looking forward to taking charge of PSG in Europe for the first time, though he'll be starting with a real baptism of fire -- a two-legged round-of-16 knockout clash against Barcelona.

Man City sailed through

Manchester City went sailing through their group campaign with five wins from six outings, the only dropped points coming via a 0-0 draw against Porto in early December.

Pep Guardiola's side topped Group C in unflustered fashion before continuing their dominant form on the domestic scene over the winter period.

City are now sitting pretty at the top of the Premier League having recently set a record for the most consecutive victories in all competitions of any top-flight club in English football history. Indeed, a routine 3-1 win over Swansea in the FA Cup last weekend saw City enter the record books by racking up their 15th win on the bounce in all domestic competitions -- a streak during which they've scored 40 goals and conceded just five.

They now face Borussia Monchengladbach in the next phase of the Champions League, with the German side likely to be quaking in their boots at the very thought of facing a team in such rampant form.

Liverpool's implosion started

Ever since Liverpool suffered a shock 2-0 Champions League home defeat against Atalanta at the tail end of November -- losing a competitive match at Anfield by a margin of more than one goal for the first time in 137 matches -- the rot set it, and their subsequent results have been all over the place.

It hasn't all been bad, but with only seven wins from 16 games, it's fair to say that Jurgen Klopp's once-dominant side have struggled since that sobering night.

Most recently, three defeats in their last five games have seen Liverpool eliminated from the FA Cup and fall 13 points behind leaders Man City in their increasingly unlikely quest to defend the Premier League title.

Now that the Champions League offers new hope, Klopp's men will be seeking to re-focus their efforts and arrest the slump with a resurgent performance against RB Leipzig in the round of 16.

Manchester United didn't make it

And where are United, the aforementioned semi-vanquishers of PSG, in all this?

Well, you'll be able to find Ole Gunnar Solskjaer's team taking on Real Sociedad in the round of 32 of the Europa League on Thursday night.

As you may recall, United were sent packing from the Champions League when they lost 3-2 against RB Leipzig -- the first time the club have been eliminated at the group stage of the competition since 2015-16, when Louis van Gaal was at the helm.