It's a cliche because it's true. With just 270 minutes (plus extra time) left to play, anyone can win the Champions League. Liverpool were in the final just a year ago and Barcelona were the last team not named Real Madrid to actually win it. Ajax and Tottenham look like the longshots but have earned their places by taking down Borussia Dortmund, Real Madrid, Manchester City and Juventus in the past two rounds. Plus, thanks to the fact that the quarterfinal and semifinal draws were made at the same time, one of them is already guaranteed to make the final.
So let's take a look at why each team will and won't be lifting the Champions League trophy in Madrid come June 1.
Barcelona: Lionel Messi's magicians
Why They'll Win: The last time Barcelona made it this far, they won the whole thing. In 2015, they'd successfully transitioned out of the "tiki-taka" era and seemed to have a clear succession plan in place: as Lionel Messi aged into his 30s and gradually declined, Neymar would just pick up the slack. Well, four years later, Neymar's gone -- and once again watching the latter rounds of the Champions League from somewhere in Paris -- and Barcelona might be more reliant on Messi than they ever have been.
Messi could have retired last summer and laid claim to owning the best club soccer career we've ever seen but instead, at age 31, he's putting the finishing touches on what might be his best year yet.
Who's leading Europe in goals scored? Messi. Assists? Messi. Through-balls? Messi. Take-ons? That's Eden Hazard, but Messi is second. Messi is the best player of all time because he's basically been the best scorer, creator, passer and dribbler on every field he's ever stepped on. Somehow, in his 15th season, that's more true than it's ever been.
Why They Won't: How do I say this? Their defense... it sucks. It doesn't suck in the grand scheme of European soccer but for a Champions League contender, it sure does. According to expected goals, which uses a variety of factors to put a historical conversion probability on every shot a team takes and concedes, Barcelona have a worse defense than five teams in La Liga alone. They don't dominate possession or press as effectively from the front as they used to, and the guys who always cleaned things up on the back end -- namely, Sergio Busquets and Gerard Pique -- have aged into their 30s.
As such, rather than leveraging the entire team into an 11-man attacking machine, manager Ernesto Valverde has decided to take his foot off the gas and ask Messi to do everything. On the whole, it's worked out: They're nine points ahead of second place in La Liga and are three games from a fifth Champions League trophy. But being so reliant on a single player -- even if it's the single greatest player -- exposes you to an injury, an off day or a particularly effective opposition game plan.
With their Messi-centric approach, the current iteration of Barcelona basically function as a more talented, better coached version of Argentina. And yeah: you already know how that story goes.
Liverpool: The complete package
Why They'll Win: Remember the helter-skelter Liverpool of the past few years? The one that could nearly blow a 5-0 lead against Roma, lose 4-1 to Tottenham and concede seven combined goals against Swansea and Bournemouth? That team is dead. Long live Simon Mignolet and Ragnar Klavan.
With a slightly less manic, teamwide ball-chasing approach, and a rearguard that now includes arguably the world's best defender (Virgil van Dijk) and the world's best goalkeeper (Alisson), preventing goals has become Liverpool's strength. Based on the number of goals conceded in domestic play -- 20 in 34 games -- Jurgen Klopp's side brings the competition's best defense into the semifinals.
The results of knockout games between relatively even teams often have little to do with who dominates space or who creates the better chances. Games typically come down to individual moments: both mistakes and bits of brilliance. After losing last year's final thanks to a Gareth Bale bicycle kick and two outlandish errors by keeper Loris Karius, Liverpool know this better than anyone. Van Dijk and Allison all but eliminate the mistakes on the defensive end, which means any moment of brilliance on the other side of the field is more likely to decide a game.
Liverpool are the only semifinalist with three players -- Mohamed Salah, Roberto Firmino, and Sadio Mane -- who have at least 18 combined goals and assists this season. They had those three players who could win a match last year, too, but now they have the defense to allow them to do it.
Why They Won't: In short, they don't have Messi.
It might seem like picking nits but deeply flawed teams rarely make it this far in the Champions League. That high-flying front three provides the unpredictability of a goal coming from anywhere across the attack, but the team still doesn't have a standout creator who can break down a defense and magic up a chance for a teammate.
Klopp has famously said "no playmaker in the world can be as good as a good counter-pressing situation," but Liverpool aren't quite pressing as aggressively as they have in the past. Plus, among Europe's top five leagues, not a single player in the Liverpool squad is in the top 50 of open-play chances created per 90 minutes.
The lack of a go-to creator hasn't really mattered just yet; they're here and they have the second-best 34-game record in Premier League history. However, some of Liverpool's worst games this season -- the scoreless home draw with City, the 1-0 road loss to Napoli -- have come against teams who made a point of sticking a branch in their gears. Klopp's side take 15 shots per game but they registered just 11 combined attempts over those two matches.
Ajax: The underdogs who play like champions
Why They'll Win: They know who they are. For whatever reason, most teams tend to shift into a slightly more conservative gear when they're away from home. (See: Barcelona and Manchester City combining for just 16 shots in their two road quarterfinal matches.) Not this team, though. Call it "youthful naivete." Call it "the legacy of Total Football," or simply call it "Hakim Ziyech has never met a shot he won't take." Whatever the reason, Ajax play the same way, no matter where they are.
They took a 2-1 deficit into the Santiago Bernabeu against the three-time defending champs and ripped off 16 shots en route to a 4-1 win. They followed that up by taking down Cristiano Ronaldo and Juventus with a thumping 2-1 victory that could have been 5-1 on a different day. Ajax don't grind out victories; they vaporize any team that's in their way.
Ajax's most recent annual revenues were £81 million. As for their fellow semifinalists: Tottenham brought in £372 million, Liverpool £455 million and Barcelona £612 million. The typical underdog knows the odds aren't in their favor, so they play a reactive style that cedes possession, limits the quality of the opposition's chances and creates space on the other end for the occasional counter-attack or set piece. But for every Leicester City and Atletico Madrid, there are countless other teams who tried something similar, failed and were swiftly swept into the dustbin of history.
The reason Ajax have made it this far -- and could go even farther -- isn't that they've played the probabilities properly and caught lightning in a bottle. No, it's this: Even though the finances say they're David, they play like they're Goliath.
Why They Won't: What happens when they can't play their game? Ajax's wins were predicated on passing over, around, underneath and through their opponents. At their best, they systematically create an overwhelming number of chances and enough of them go in. Neither Real Madrid nor Juventus are teams that will automatically dominate possession, and neither side has an effective, swarming counter-press. So, they both allowed Ajax to play the way they want to play.
In three of the four games against those two sides, Barca-bound Frenkie de Jong completed more passes than anyone else on the field. What happens if an opposition press, much like Tottenham did with Chelsea's Jorginho, removes him from build-up play? What if their opponents dominate possession and field position? Can Ajax shift into a more vertical, counter-attacking approach? Or do they need the ball in order to generate enough shots to score?
Of the four remaining teams, Ajax have to answer the most questions.
Tottenham: The never-say-quit collective
Why They'll Win: They have one of the best finishers in the world. "But," you say, "Harry Kane is out for the year!" To which I respond: meet this guy.
Forget the emotional torture of VAR. Briefly ignore Pep Guardiola's inability to either wear his hood properly or coach Manchester City beyond the Champions League quarterfinals. City vs. Tottenham was absolute madness but beneath all of the narratives and emotions, there's a simple story: Tottenham converted a high percentage of their chances and City didn't. They each scored four goals but according to FiveThirtyEight, City created 3.7 expected goals worth of chances, while Tottenham registered just 1.6.
Part of that is luck and part of it is that they have Son Heung-Min. Most players over time score roughly around the same number of goals as expected but a handful, like Messi and Son, consistently finish at a much higher rate. Over the past six years, per the website Understat, Son has scored 53 goals in domestic play on just 35.75 expected goals. He's equally adept with either foot -- 30 with his right, 21 with his left -- and if Tottenham manage to lift their first-ever European Cup, they'll be two of the biggest reasons why.
Why They Won't: Son is suspended for the first leg against Ajax and they're simply running out of players!
Making it this far is an incredible achievement for a club that hasn't signed a new player in either of the past two transfer windows, a first for the Premier League. Mauricio Pochettino has pieced together various lineups that barely contain anything that could legitimately be described as "a midfield" but the club's lack of ambition in squad-building, coupled with a number of key injuries and the regular attrition of a long season, has left them terribly thin.
Against Man City on Wednesday night, Moussa Sissoko -- once an outcast whom Pochettino has been forced to reintegrate into the midfield as a key cog -- went off injured in the first half and was replaced with Fernando Llorente, who is 34 years old and absolutely not a midfielder. The team has plenty of extra central defenders and fullbacks, but there's close to no cover remaining in the midfield or the attack.
Currently one point clear of fifth-place Chelsea (and with a game in hand), Spurs have five close-to-must-win Premier League games left, in addition to the two legs against Ajax and a potential European final. Ajax, Barcelona,and Liverpool all enter this stage of the tournament with what are close to their first-choice XIs fully intact. Even in the best case scenario, Tottenham just won't be able to say the same.