Guarani among scrapping underdogs in Copa Libertadores knockout rounds

South America's top teams went into the group phase of the Copa Libertadores some three months ago and come Thursday night, only eight of them will be left standing. This is the week when the competition defines its quarterfinalists.

One team is already there. Tigres of Mexico have played both legs of their tie against Universitario Sucre of Bolivia and came through 3-2 on aggregate.

The Bolivians put up stiffer resistance than expected but the favourites came through. The same thing may not happen in all of the seven other clashes, however, as the underdogs rose to the occasion in last week's first leg matches. This was clearest in the two biggest victories of the week; 2-0 triumphs for both Emelec of Ecuador against Atletico Nacional of Colombia and Guarani of Paraguay over Corinthians of Brazil.

How did it happen, and can the favourites come good in the return game?

Corinthians vs. Guarani -- Wednesday, May 13

It seems clear that Corinthians and the Brazilian football community in general under-estimated Guarani. It was perhaps overlooked that in both the last two years, Paraguayan teams reached the final of the Libertadores and that Guarani are no worse than Nacional or Olimpia, runners-up in 2014 and 2013 respectively.

Guarani may not be seen as one of Paraguay's giants, but they are a well-run club where Spanish coach Fernando Jubero seems to be doing a fine job. They have also grown during the course of the competition. A key development was the switch to a back three, a formation which is bringing the best out of two experienced defenders. Recent signing Ruben Maldonado, 36 and with eight years in Italian football on his CV, is a centre back in the best rugged and uncomplicated Paraguayan tradition.

Julio Cesar Caceres, 35, is looking good in the middle of the back three, a position which highlights his reading of the game and timing in the tackle.

Up front, dangerous target man Federico Santander is well-supported by the elusive wing play of Julian Benitez.

The vulnerable space against them is down the flanks, outside the back three. If Corinthians can get behind the wing backs then they should be able to supply star centre forward Paolo Guerrero with some chances.

This was Corinthians' failing in the first leg. Their best chance came when left back Fabio Santos cut in to send a shot against the post. They looked most lively in the closing stages when little winger Malcom came off the bench and used his pace down the right.

For most of the game, though, they were slow and static. The big task of coach Tite is to get this right for Wednesday night, to get his team passing the ball and moving into space to find the holes in the Guarani defence. Over-reach, though, and they could leave themselves open to the bursts of Santander and Benitez on the break -- and if Guarani score once, Corinthians will need four. Tite's team, which started the year so well, have gone off the boil in recent weeks. Early elimination from the Libertadores would be a serious blow. There is plenty at stake, then, in Sao Paulo.

Atletico Nacional vs. Emelec -- Thursday, May 14

After a fine start, Emelec stuttered into the knockout phase. The trajectory of Atletico Nacional was the reverse. The momentum, then, seemed to be with the Colombians until they fell to a 2-0 defeat last week in Manta, where Emelec are staging matches while their stadium in Guayaquil is undergoing building work.

The Ecuadorian champions have always looked a threat in this Libertadores. They have acquired plenty of international experience in recent years, have a well-defined model of play and in Miller Bolanos, they have one of the most dangerous strikers on the continent. Indeed, the team's form has reflected his own; Bolanos can blow hot and cold. When he has been on form, though, the team have usually won.

He was certainly on his game last Thursday but his task was eased by a key absentee in the opposing side. Nacional coach Juan Carlos Osorio is a fascinatingly restless figure, frequently changing his line up and formation, and often experimenting by trying out players in different positions. But a fixed point in his team is lanky centre back Alexis Henriquez.

The pair previously worked together at another Colombian club, Once Caldas, and when Osorio took over at Nacional in 2012 he wasted no time in sending for Henriquez. The defender, now 32, is not especially fleet of foot but he is mentally sharp. Osorio spends much of the game writing out little notes for his players which are ferried to Henriquez, who passes on the necessary instructions. He is the brains of the team and the organiser of the defence -- but he missed Thursday's game through injury.

Both goals that Nacional conceded were very similar; a pass from deep split the centre backs and put a striker clean through on goal. Had Henriquez been present, this is a door he would have expected to slam shut.

Perhaps the biggest surprise of Thursday's first leg was Nacional's inability to expose the lack of pace at the heart of the Emelec defence. Neither Gabriel Achilier nor the veteran Jorge Guagua are quick but Nacional's attacking stars had a collective off day.

Centre forward Luis Carlos Ruiz, hero of the previous game against Libertad of Paraguay, cut a frustrated figure, at odds with himself and the referee as nothing went right for him.

Attacking midfielder Yulian Mejia, the revelation of the competition so far, left the field injured after an ineffective hour. In hindsight perhaps Orlando Berrio should have started -- he may have unsettled the Emelec defence with his pace and power down the right.

In Thursday's rematch, of course, Emelec will be able to sit deep and protect their lead, thus reducing the space their defenders have to cover.

Atletico Nacional will need some guile to break them down. Can they find it or can Bolanos undo them on the break? It promises to be a fascinating way to end the second round of the Libertadores.