Chelsea may have been relieved to avoid European giants Bayern Munich and Real Madrid in the semi-final draw, but tonight's opponents are currently one of the most in-form teams in Europe.
As the Blues prepare to face Atletico Madrid in the first leg at the Vicente Calderon, we take a detailed look at their opponents.
Route to the semi-finals
Atletico are the only unbeaten side left in this season's competition after they eased through a group containing Zenit St Petersburg, Porto and Austria Wien.
They overcame seven-time winners AC Milan in the last 16 before causing a huge upset by knocking out Spanish rivals Barcelona in the quarter finals.
Atletico possess one of the most feared strikers in the world in Diego Costa who is ironically rumoured to be Jose Mourinho's prime target for Chelsea this summer.
The Spanish international striker has scored 35 goals in all competitions and has been integral to the club's domestic and European success this season.
Manager Diego Simeone recently admitted that it "wouldn't surprise him" if Costa completed a sunmmer move to a "financially powerful" side.
Best remembered in Britain for his role in David Beckham's sending off during the World Cup in 1998, Coach Diego Simeone has transformed Atletico since taking over in December 2011.
The Argentinian manager has won the Europa League, European Super Cup and Spanish Cup and has taken them to the Champions League semi-finals for the first time in over 40 years.
A mix of defensive discipline and counter-attacking flair has propelled the Spanish side to the top of La Liga ahead of Barcelona and Real Madrid with just four games to go.
Simeone said his side will "take advantage of our strengths and disguise our weaknesses" when they face the London side.
Atletico will have a tough job keeping hold of the 43-year-old who is believed to be on Manchester United's shortlist to replace David Moyes this summer.
Chelsea will be greeted by a hostile atmosphere when they walk down the tunnel at the Vicente Calderon stadium tonight.
The 50,000-seater stadium is renowned for its noise and Atletico have never lost at home to an English side.
Chelsea's progress in the competition may be determined by how they contend with the environment.
Will Thibaut Courtois play?
Atletico Madrid's on-loan goalkeeper Thibaut Courtois will be allowed to play against his parent club in the semi-final.
It had been reported that Courtois would only be allowed to play if the Spanish side paid Chelsea a substantial sum, but Uefa released a statement last week clearing the goalkeeper to play.
Courtois later admitted he was very relieved to have been given permission to take part.
Chelsea will have to produce something special if they are to beat the 21-year-old Belgian who has kept 26 clean sheets in 49 games this season.
Watch Atletico Madrid v Chelsea on ITV and itv.com/football at 7.30pm tonight