It might seem like there are clear favorites in each Champions League quarterfinal matchup, but based on the betting odds, there's only about a 1-in-5 chance that Real Madrid, Bayern Munich, Paris Saint-Germain and Barcelona all advance.
That's because a lot can happen across 180 minutes of knockout soccer.
Optimal tactics, systems, strategies, roster quality, talent depth are the things that win you league titles. But over just two matches, individual performances and stylistic dynamics matter more than anything else.
Two underwhelming matches from your star winger, and suddenly you're sitting at home. A blinder from your backup fullback? Here's your ticket to the semifinals.
So, ahead of the first legs of the four remaining matchups kicking off Tuesday and Wednesday, let's take a look at the players, people and philosophies that might have an outsize effect on who punches through to the semifinals.
Here are your eight X factors for the UEFA Champions League quarterfinalists: