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Europe's elite football club competition - the Champions League - is back 150 days after it was put on hold because of coronavirus.

Before the next stage of the competition can go ahead, English clubs Manchester City and Chelsea still have to finish their Round of 16 matches.

Then in a change to this year's competition, it's a straight knock-out in a behind-closed-doors tournament in Portugal.

Here are five things to watch out for.

Normally the knock-out rounds of the Champions League feature two games: home and away. This year, because the tournament has already been delayed by the pandemic, things need to be wrapped up quickly.

That means the final few games of the competition will see a 'final-eight' mini-tournament take place in Portugal from 12 August.

Games will be played over a single game rather than a home and away leg. The winner takes it all with extra time and penalties if matches are tied after 90 minutes.

Messi and Ronaldo might be looking ahead to retirement, but they are still the best in the business.

It's often debated who is actually better, and whether it's Messi or Ronaldo who should be described as the ultimate GOAT (greatest of all time).

Both players are still performing at the very top level, despite being in their 30s. Ronaldo has said in interviews that he feels in good enough shape to play until he's 40 though!

"I could quit in a season's time, or I could play until I'm 40 or 41. Everything depends on how I feel, my motivation. Because physically, it's never going to be a problem. I look after myself really well, and I'm quite certain that I could play until I was over 40. It won't last forever, but I feel strong enough to go on winning important trophies," he said.

Even at their age, both Messi and Ronaldo can still influence the outcome of this year's Champions League too.

Ronaldo's Juventus and Messi's Barcelona are still in the competition. Provided they keep winning, the old rivals could face each other in the semi-finals.

The one game knock-out format could be good news for Manchester City, who are hoping to win the competition for the first time in their history.

But to get to the final stages in Portugal, they first have to beat record Champions League winners Real Madrid on Friday.

The Citizens have a one-goal advantage ahead of their second leg home game with Real, and if they win, they'll face Lyon or Cristiano Ronaldo's Juventus in the quarter-finals.

City boss Pep Guardiola has won the competition twice as manager of Barcelona, but the Champions League is the one trophy he's failed to win while managing City.

The end of Chelsea's season has represented the rollercoaster that is football.

The Blues finished the Premier League season in fourth, qualifying for next season's Champions League but then lost the FA Cup final to Arsenal.

Now Frank Lampard's side have to try to turn around a 3-0 first-leg defeat to Bayern Munich. The Blues lost the first match at Stamford Bridge back in March.

To make matters worse the German league champions have not lost a competitive match since 7 December!

For many teams the Europa League is a pathway to the Champions League, winning the competition means automatic qualification.

Premier League Wolves, who missed out on European qualification through their league position, will want to get as far as possible in the competition.

They beat Greek champions, Olympiacos, 2-1 over two matches to progress to the quarter-finals stage.

Next up, they'll face Spanish side Sevilla for the chance to make it into the semi-finals of the competition.

Meanwhile Manchester United, who finished third in the Premier League, have already qualified for next season's Champions League.

However, the Red Devils will want to win the Europa League, as it's over 30 years since United went three seasons without winning a trophy.

Ole Gunnar Solskjaer's side beat LASK 7-1 on aggregate and are through to the Europa League quarter-finals where they will face Danish team FC Copenhagen in a one-off knock-out game in Germany.

For Scottish runners-up Rangers, any dreams of winning the Europa League are over for this season after they were beaten 4-1 by German side, Bayer 04 Leverkusen, over two matches.