Real Madrid and Atletico Madrid are familiar foes, having faced each other in two of the last three Champions League finals - Real coming out on top in both.
The city rivals will meet for a place in this season's final in Cardiff, with the first leg of thir last-four tie taking place at the Santiago Bernabeu on Tuesday.
Although the head-to-head record between the sides reads in favour of Real, Atleti can never be underestimated or written off.
Star players Cristiano Ronaldo and Antoine Griezmann could hold the key, the forwards having scored seven and five goals respectively in this season's competition.
Here, we take a look at some of the key Opta stats ahead of this week's first leg.
- This is the fourth season in succession that the teams have met in the Champions League knockout phase and Atletico are yet to come out on top against their rivals in that run.
- This year's Madrid derby is a repeat of the 2013/14 and 2015/16 finals. Real won on both occasions, claiming a 4-1 win and a penalty shootout victory respectively to lift the trophy.
- Atletico are competing in the semi-finals of the Champions League for the third time in the last four years, having never before gone as far in the competition.
- Meanwhile, it is the seventh year in a row that Real have been among the semi-finalists, their current sequence a record for the tournament.
- Defensive stability will be key if Atletico are to progress, with Simeone's men having kept 27 clean sheets in 46 Champions League games under his leadership, more than any other club in the competition during that period.
- The first goal is usually vital in knockout games but Real will not be concerned if Atletico score first. Real have conceded first in all four knockout games this season but have won every match regardless.
- Ronaldo became the first player to reach the 100-goal mark in the Champions League during the quarter-final against Bayern Munich, a tie in which he scored five goals.
- He has scored 0.75 goals per game in the knockout stages of the Champions League (49 in 65 games), which is a better ratio than his record in the group stages (0.71).
- Ronaldo's equivalent at Atletico is Griezmann, who has scored or assisted half of his team's goals in the Champions League since last season (12 goals and 3 assists).
- Expect a slow start from Real in the semi-final, as none of their last 11 Champions League goals have been scored in the first half.