The Champions League semifinals are perhaps the most tactically fascinating matches in football -- the best club sides in the world competing over two legs. With no Barcelona and no Bayern Munich, this season's final four offer more tactical diversity than usual, and these could be two thrilling contests. Here's how each side might play to secure their place in Cardiff next month.

REAL MADRID vs. ATLETICO MADRID

Real Madrid: Use aerial threats

The absence of the injured Gareth Bale from Real Madrid's starting XI means their system will change slightly, but significantly. Although Marco Asensio is an option, Zinedine Zidane seems more likely to use Isco, a playmaker who will drift inside more, acting as a fourth midfielder rather than a third forward.

In turn, this will affect the way Real's attackers operate. Cristiano Ronaldo usually has license to stay up front rather than track back anyway, but the extra balance provided by Isco means he'll have even greater freedom to focus on attacking. Therefore, it's likely he and Karim Benzema will effectively form a front two, with Real concentrating heavily on getting crosses into the box.

Ronaldo has increasingly evolved into a No. 9 rather than a left-winger and, against an Atletico side likely to sit deep but keeping compact between the lines, the Portuguese forward is likely to spend long periods in the box, simply waiting for service.

Usual Atletico right-back Juanfran, often the unfortunate fall-guy in recent defeats to Real, is out injured -- as is his most obvious replacement, Jose Gimenez. This means Diego Simeone may be forced to field midfielder Thomas Partey out of position, which would invite plenty of Real crosses towards Ronaldo. Lucas Hernandez, a natural defender but more comfortable on the left, is the alternative.

Atletico Madrid: Press laterally to isolate Real's full-backs

Over the past four seasons, Atletico Madrid have become renowned as European football's most well-organised, tactically disciplined side -- largely because of their approach without the ball. And while Diego Simeone's men usually sit deep and invite pressure against major opposition, their excellent pressing shouldn't be underestimated. But whereas other teams press vertically, pushing the opposition backwards, Atletico are more about pressing laterally and boxing the opposition into the touchlines.

This could be a valuable approach against Real Madrid, whose width comes almost exclusively from their two rampaging full-backs, Dani Carvajal and Marcelo. Both were outstanding in an attacking sense during the quarterfinal victory over Bayern Munich, but will find things tougher against an Atletico side who will set a trap, then pounce.

Atletico will set out in a narrow shape, inviting passes out wide to Real's full-backs. But this will probably act as the trigger for Atletico's press, and Marcelo and Carvajal will quickly find themselves shut down by Atletico's wide midfielders, with the full-back blocking any pass down the line, and Atletico's centre-forwards blocking passes back to the centre-backs. The speed and intensity of the press is frightening, and Real may struggle.

This will, of course, be combined with quick transitions to launch counter-attacks immediately, most obviously through star man Antoine Griezmann. This tried-and-tested approach has worked many times before, but might be particularly useful here.

MONACO vs. JUVENTUS

Monaco: Play cut-backs

Monaco's free-flowing, cohesive attacking football under Leonardo Jardim has won plenty of admirers across Europe this season, and one of the key parts of Monaco's gameplan is the fact they've always got so many players in goal scoring positions.

Indeed, while they can depend upon the speed of Kylian Mbappe and the penalty box poaching of Radamel Falcao, Monaco share the goals around excellently. They've already hit 95 in Ligue 1 this season, and nine separate players have scored six or more. Monaco get their midfielders forward into goal scoring positions, and that could prove crucial here.

In particular, this might trouble Juventus when it comes to Monaco playing cut-backs. The French league leaders tend to play with plenty of width, but will be discouraged from crossing the ball because Giorgio Chiellini and Leonardo Bonucci are so dominant inside their own penalty box.

However, Juve are without the suspended Sami Khedira, the midfielder who occupies the zone in front of the back four so effectively. Instead, they'll field Miralem Pjanic and Claudio Marchisio there -- two naturally creative players who, while tactically disciplined, aren't quite so solid defensively. Therefore, a gap may open up in front of Juve's impenetrable back four.

Monaco play cut-backs effectively. In the victory over Manchester City, for example, their opening two goals came from a low left-wing cross to Mbappe, then a classic cut-back to the onrushing Fabinho. They get numbers around the penalty box, and seek to overwhelm the opposition.

Falcao and Mbappe are the big threats, but the supporting cast might prove crucial.

Juventus: Play balls in behind Benjamin Mendy

One of Monaco's key attacking threats is their rampaging left-back Benajamin Mendy, who has managed five league assists this season and three more in European competition. Juventus must track his forward runs carefully, but may also seek to exploit the space in behind him.

Indeed, while Mendy flies forward regularly, Monaco aren't great at covering for him, and Juventus might be able to exploit his advanced positions with quick counter-attacks. There are three ways they might be able to do this.

First, while right-winger Juan Cuadrado will have some responsibility for following Mendy towards his own goal, he's also excellent at breaking quickly. His runs into the space outside Barcelona's defence were a crucial feature of Juve's quarterfinal performance and, although his final ball was disappointing, Juve will probably take advantage of his speed again.

Second, there's the runs of Gonzalo Higuain, who has seemingly become quicker in recent seasons, and more capable of working the channels to sprint onto balls played behind the opposition. Juve don't tend to play in such a direct manner, but against a Monaco side throwing numbers forward, that could be an interesting approach.

But the third player who may exploit the space in behind Mendy is the most intriguing -- Paulo Dybala. He's fielded in a central position, but constantly drifts towards the flanks and, as his superb opener against Barcelona demonstrated, he's very much capable of working the inside-right channel. The more Mendy advances, the more Juventus might find space to break into.