Champions League: What has happened so far this season?

Robert Lewandowski, Erling Braut Haaland, Adrian and Josip Ilicic have all played big roles - some good and some bad - in the Champions League this season

The Champions League returns on Friday - and it feels like a lifetime ago since the group stages and early knockout matches.

The remaining four last-16 ties will be played this weekend, with the rest of the tournament then happening in a series of one-off games in Lisbon.

What has already happened this season? BBC Sport recaps some of the main storylines.

A new star emerges - while a veteran enjoys his best season

Robert Lewandowski and Erling Braut Haaland — Robert Lewandowski has scored a Champions League goal every 48 minutes this season, while Erling Braut Haaland averages one every 55 minutes

Two players have reached double figures in the Champions League this season - after just six and eight games respectively.

Bayern Munich striker Robert Lewandowski has scored a remarkable 11 goals in six matches.

He netted against Red Star Belgrade, twice against Tottenham, twice and once against Olympiakos, four times in Belgrade, was rested against Spurs and then scored once against Chelsea.

The 31-year-old Poland international is enjoying the best season of his career, with 51 goals in 43 matches for Bayern - including one every 81 minutes in the Bundesliga.

He is one clear of Erling Braut Haaland, who had maybe the most spectacular start to Champions League life of any player.

Haaland, who only turned 20 in July, scored an unbelievable 10 goals in eight games (one every 55 minutes) - despite being knocked out of the tournament twice.

He became the first player to score eight goals in his first five appearances in the Champions League - but Red Bull Salzburg went out in the group stages with defeat by Liverpool, despite scoring 16 goals. Only three teams scored more in this season's group stages.

Haaland then joined Borussia Dortmund, and struck twice against Paris St-Germain to reach 10 goals in seven games, four games quicker than any other player in the competition's history.

The Norway international did not score as they went out in the second leg - but he has still managed as many Champions League goals this season as Barcelona.

He will quite possibly finish as second top scorer, with the changed format meaning teams only have a maximum of three or four matches left. The next top scorers still in the tournament are Bayern's Serge Gnabry and Dries Mertens of Napoli, who have six each.

Liverpool knocked out - in a game with bigger ramifications

'Poor decision' to stage Atletico game in March

Liverpool were hoping to reach a third consecutive Champions League final but fell to Atletico Madrid in extra time in the last 16 on 11 March.

That was the first time they competed in the tournament without reaching the final since 2014-15.

Trailing 1-0 from the first leg, Liverpool took the tie to extra time at Anfield thanks to Georginio Wijnaldum's goal, before Roberto Firmino gave them the aggregate lead. An Adrian error then gifted the first of three extra-time goals to Atletico, who won 3-2 on the night and 4-2 on aggregate.

But the game may well go down in history for different reasons. Some 3,000 Atletico fans flew over from Madrid, despite the city having almost half of Spain's coronavirus cases at the time and being in partial lockdown.

That was the last major match played in England for more than three months. Two days later, the Champions League and Premier League were among the many competitions suspended.

A Sunday Times report claimed 41 deaths in the Liverpool region were linked to the game as data gathered from millions of volunteers found rates of cases locally "increased several-fold".

Steve Rotheram, the mayor of Liverpool City Region, called for an investigation into whether the game should ever have happened.

There should be an inquiry - Rotheram

Entertainers Atalanta come back from the brink

Atalanta forward Josip Ilicic — Atalanta's win in Valencia was played behind closed doors - a sign of things to come

Atalanta were hoping to make an impact in their first Champions League campaign after scoring 77 goals in Serie A, more than any other team, to finish third the previous season.

But they looked doomed after losing their first three games - including a 4-0 defeat at Dinamo Zagreb and 5-1 thrashing by Manchester City - and picking up one point in their fourth.

But they beat Zagreb 2-0 and Shakhtar Donetsk 3-0 to become the first side to qualify for the next round after losing their opening three matches since Newcastle United in 2002-03. They are also the first debutants to reach the knockout stages since Leicester City in 2016-17.

They showed why they are considered one of Europe's most entertaining teams - they scored 98 goals in Italy's top flight this season - with an 8-4 aggregate win over Valencia in the last 16.

They won 4-1 at the San Siro in Milan, their Champions League home because their ground does not meet Uefa requirements. Josip Ilicic then scored four goals in Spain in a second leg played behind closed doors - as the rest of the tournament will be.

This was another tie with big ramifications, with the mayor of Bergamo - where Atalanta play - calling the first leg "a biological bomb" as the city went on to become one of the worst-hit areas in Europe.

Ajax knocked out - despite some memorable games

Donny van de Beek's 55th-minute goal gave Ajax a 4-1 lead at Chelsea, before the London club fought back to draw

Ajax's run to the brink of last season's Champions League final - before Lucas Moura's last-gasp winner for Spurs in the semi-finals - was a story for the romantics.

But they were predictably asset-stripped by Europe's elite, with star youngsters Frenkie de Jong and Matthijs de Ligt going to Barcelona and Juventus respectively.

And they went out this season despite having a record (10 points and a goal difference of six) that would have taken them through in five of the eight groups.

They beat Lille and Valencia 3-0 and led 4-1 at Chelsea after an hour, before ending with nine men and drawing 4-4.

A home defeat by Valencia in their final group game meant they dropped into the Europa League, from which they have since been eliminated.

German teams 17-3 London sides

Serge Gnabry became the first player to score four goals in the second half of a Champions League game when Bayern won 7-2 at Spurs

Bayern Munich and RB Leipzig have enjoyed their matches against London sides this season - with five wins out of five, and the last-16 second leg between Bayern Munich and Chelsea yet to come.

Gnabry's six Champions League goals this season have all come in two games - four in a 7-2 win at Spurs and two in the 3-0 first-leg win over Chelsea.

Bayern Munich beat Spurs 3-1 in their final group match, with both sides qualifying for the last 16. Sadly for Spurs, they met another German team at that stage - with Leipzig winning 1-0 in England and 3-0 at home.

Leipzig will be missing one of their goalscorers when they play Atletico Madrid in the quarter-finals. Timo Werner has already linked up with Chelsea, although he cannot play for his new club in their second leg against Bayern, or any game this season.