TEAMtalk outlines the key battles as Barcelona prepare to face arch-rivals Real Madrid at the Nou Camp in this weekend's El Clasico.

Madrid ended Barca's three-year hold on the Primera Liga crown last season but have made a faltering start to their title defence.

Jose Mourinho's men have lost two of their opening six league games to fall eight points behind the Catalans, making this game a must-win even though it comes so early in the season.

Here, TEAMtalk takes a look at the key battles.

XAVI v XABI ALONSO

For all the talk of the shoot-out between Lionel Messi and Cristiano Ronaldo, the match may be decided by the two key midfielders on show.

Xavi sets the tempo for Barca, controlling the speed of their tiki-taka approach, determining when to up the pace and when to sit back and retain possession. The 32-year-old also has the eye for a killer pass, the kind from which Messi has so often profited.

Alonso offers Madrid something a little different. The former Liverpool man plays a touch deeper than Xavi, allowing the more offensive midfielders such as Mesut Ozil and Angel di Maria to bomb forward. He is also more physical and will be expected to break up the Barca attacks, with Xavi one of the players he will pay particular attention to.

JAVIER MASCHERANO v CRISTIANO RONALDO

Stand-in Barcelona centre-back Mascherano will have his work cut out to contain Ronaldo. While Madrid have not fired as a team this season, the Portugal star has simply continued his blistering form since moving to Spain. Back-to-back hat-tricks in the last week, against Deportivo La Coruna in the Primera Division and Ajax in the Champions League, improved his phenomenal record at the Bernabeu to 158 goals in 154 appearances and he will again offer the biggest threat to the home side.

Midfielder Mascherano has performed admirably at the back in the absence of regular defenders Gerard Pique and Carles Puyol, with Barca conceding just five goals in six league games - but the Argentina international will need to be at his best on Sunday night.

LIONEL MESSI v PEPE

Messi has 10 goals this season and remains Barca's stand-out performer. The Argentinian's feats almost slip under the radar now as his excellence is taken for granted but his record remains remarkable. He scored 50 goals last season to win the Pichichi - four more than Ronaldo - and stepped up admirably following the long-term injury suffered by the club's chief out-and-out striker David Villa.

If anyone can unsettle Messi it is likely to be Pepe. The Brazil-born Portugal defender is renown for an uncompromising approach which has often seen him fall foul of opponents and referees alike. But he possesses great athleticism, a pre-requisite for facing Messi. If he can shackle Barca's dangerman it will give Madrid the platform they need to seek a much-needed win.