Real Madrid
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AC Ajaccio
7:00 PM UTC
Leg 2Aggregate: 3 - 0
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Rotherham United
Shrewsbury Town
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Sporting Kansas City
Columbus Crew SC
10:00 PM UTC
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 By Michael Cox

Key round of 16 battles

This has been the most fascinating World Cup in recent memory and we haven't even reached the good stuff yet. With the also-rans eliminated, the next four days sees eight intriguing knockout ties featuring some extremely impressive teams and some exciting individual battles ...

Brazil vs. Chile: Luiz Gustavo vs. Arturo Vidal

Luiz Felipe Scolari has changed the structure of his Brazil side since last season's Confederations Cup, preferring to use Neymar behind the central striker Fred rather than cutting inside from the left. Oscar has made the reverse switch and, while Brazil are still roughly a 4-2-3-1, it means they're now playing with two central forwards, rather than three midfielders.

Therefore, Brazil could be overrun in midfield, and there's every chance Chile will be the side to expose them. Marcelo Diaz has done a fine job at the base of the midfield trio and Charles Aranguiz has powered forward into attack, but this could be a game for Arturo Vidal. He'll press Luiz Gustavo and storm past him when Chile win possession, so it's up to the Brazil man to track those well-timed bursts into the box.

Colombia vs. Uruguay: Mario Yepes vs. Edinson Cavani

The absence of Luis Suarez -- you might have heard about the reason -- is a huge blow to Uruguay but they have one of the world's most dangerous strikers ready to replace him. Cavani found life difficult in the 3-1 defeat against Costa Rica when Suarez was out injured but that was partly because Diego Forlan, playing the supporting role, was terrible at connecting him with the rest of the side.

Uruguay need Cavani to step up in the absence of Suarez.

Oscar Tabarez will hopefully support Cavani with a more mobile midfielder -- perhaps Nicolas Lodeiro -- and Cavani should be allowed to stay upfront and battle against the centre-backs. Yepes has enjoyed a decent tournament so far, but hasn't yet faced a striker who is seriously dangerous in the channels, and if Cavani positions himself on the outside and tries to sprint in behind, Colombia could have serious problems.

Netherlands vs. Mexico: Louis van Gaal vs. Miguel Herrera

This is a clash between two of the most impressive managers of the competition so far, both of whom have favoured a three-man defence. Van Gaal switched to a 4-3-3 for the second half of the 3-2 win over Australia, but Herrera has stuck solidly to his 3-5-2 system.

The major question is about Van Gaal's formation choice. He tends to favour the use of a spare man, and therefore against Mexico's two mobile forwards there's no reason for him not to continue with the back three. This should be 3-5-2 versus 3-5-2.

These matches often become rather slow and stale, however, with the forwards struggling to find space, and the wing-backs running up and down the line with one another. Which manager will have the bravery to tweak his system to find an advantage?

Costa Rica vs. Greece: Christian Gamboa vs. Jose Holebas

Costa Rica depend heavily upon their two full-backs for width. Gamboa set up Joel Campbell's crucial equaliser against Uruguay, while Junior Diaz provided a brilliant cross for Bryan Ruiz's headed winner against Italy.

It's Gamboa who will attack more down the right, however, and he should fight himself in direct combat with Jose Holebas, who is one of the most attack-minded full-backs around. He was particularly good in Greece's narrow win over Ivory Coast because of his fearless desire to dribble out from the back. Their duel could be very interesting.

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France vs. Nigeria: Ahmed Musa vs. Mathieu Debuchy

Nigeria's star performer in their 3-2 defeat to Argentina was Musa, the left-winger who showed his trickery and efficiency in possession with two well-taken goals. His all-round performance was excellent too and he gave Argentine right-back Pablo Zabaleta a very difficult game, particularly as he was comfortable receiving the ball on the half-turn and leaving his man behind.

Debuchy is a strange player, whose performances seem to oscillate between excellent and terrible. He hasn't faced a tricky left-winger in this competition so far, and his defensive skills will be put to the test. It's not inconceivable, though, that Didier Deschamps could opt for Bacary Sagna at right-back instead after the Manchester City-bound man had a solid game against Ecuador.

Germany vs. Algeria: Benedikt Howedes vs. Sofiane Feghouli

Germany are unquestionably the superior side, but this individual battle should be good. In blunt terms, it's Germany's worst player against Algeria's best player.

Howedes, in fairness, simply isn't a left-back, but his limitations are harming Germany's game. In the 1-0 win over the USA, his teammates looked desperate not to pass to him, while the opposition didn't bother to mark him. He's not particularly comfortable on the ball, and because he's accustomed to playing at centre-back, he doesn't like being forced to turn and sprint.

That could be a serious problem against Feghouli, an excellent counterattacking playmaker. He's Algeria's first port of call when they break forward from defence, and if he bypasses Howedes quickly, neither Mats Hummels nor Per Mertesacker are entirely comfortable against pace either.

Argentina vs. Switzerland: Javier Mascherano vs. Xherdan Shaqiri

Shaqiri's performance in the 3-0 victory over Honduras was one of the best individual displays of the tournament. Or to be more precise, it was part of the best "partnership performance" of the tournament, with Shaqiri linking seamlessly with lone striker Josip Drmic throughout.

Shaqiri's hat trick vs. Honduras was the second of the World Cup.

There were two obvious plans: either Drmic could come short, and provide knock-backs for Shaqiri to shot from range, or Drmic would move into the channels and Shaqiri would slip the ball through the defence. They combined 13 times, and seven of those passes were followed by a shot, including two of Shaqiri's goals of a fine hat trick.

Therefore, there's a huge onus upon Javier Mascherano to stop Shaqiri finding so much space between the lines. It won't be difficult to prevent him finding as much room as he enjoyed against Honduras, but if Drmic and Shaqiri are on song again, they'll cause real problems.

Belgium vs. USA: Eden Hazard vs. Fabian Johnson

USA's best performer so far has been Johnson, who has impressed with his relentless forward running from right-back. His attacking play won the crucial corner that led to John Brooks' winner against Ghana in the opener, and while many expected him to be more cautious against Cristiano Ronaldo, Johnson wasn't having any of it -- he continued to charge forward down the right, ensuring that was the game's key zone.

Against Belgium, he'll be up against the tricky Eden Hazard, who will happily seize upon any counterattacking opportunities down his side. But Johnson's attacking play could be vital, and will be especially noticeable against a side lacking top-class full-backs.

Michael Cox is the editor of and a contributor to ESPN FC. Follow him on Twitter @Zonal_Marking.