El Clasico: 5 things that will decide Barcelona v Real

Barca head into this weekend's return fixture sitting pretty at the top of La Liga, some 10 points clear of Madrid.

But can Zinedine Zidane's side spring a surprise at Camp Nou, or will the Barca juggernaut roll on?

Here are five factors that could determine the outcome of the latest instalment of club football's biggest game.

'The FIFA Virus'

Lionel Messi, Luis Suarez and Neymar may sometimes give the impression that they are superhuman, but even the most feared front three in history are not immune to 'the FIFA Virus'.

All three members of Barcelona's 'MSN' made the arduous journey to and from South America during the international break for World Cup qualifiers.

Messi and Suarez both played 180 minutes for Argentina and Uruguay respectively, and while Neymar was suspended for Brazil's second game against Paraguay, he featured in their 2-2 draw with Suarez's Uruguay.

By contrast, Real Madrid's 'BBC' - Gareth Bale, Karim Benzema and Cristiano Ronaldo - enjoyed a restful two weeks.

Ronaldo was withdrawn after an hour of Portugal's low-key friendly win over Belgium after playing the full 90 minutes against Bulgaria, while Bale and Benzema were spared international duties altogether.

Believe it or not, Barcelona's stellar attacking triumvirate are human - and humans suffer from jetlag.

So if fitness and freshness becomes a decisive factor on Saturday, 'BBC' should have the edge on 'MSN'.

The Champions League looms over a title race that is already run

Even if Madrid were to upset the odds and inflict a first home defeat of the season on their sworn enemy, it seems implausible that Luis Enrique's all-conquering team will not retain their domestic title.

The league season is a write-off for Zinedine Zidane and his players – so they could be forgiven for having one eye on Wednesday's Champions League quarter-final first-leg meeting with Wolfsburg.

In the context of another treble-chasing season, Tuesday's Champions League clash with Atletico Madrid is more important to Barca than the Clasico.

Of course, neither Luis Enrique or Zidane will dare field what could be perceived as a 'weakened' team at Camp Nou, but this Clasico may be played at a less frantic pace than usual.

And that would surely suit Barca's artists - Messi, Sergio Busquets, Andres Iniesta et al - better than the likes of Ronaldo and Bale, who thrive on transitions and high-speed counter-attacks.

Sergio Ramos' back

Madrid captain Ramos withdrew from Spain's squad for their friendly against Romania with a back problem, prompting fears he could miss the Clasico.

The centre-back trained this week and has been named in Zidane's 19-man squad for the trip to Catalonia.

Ordinarily, Zidane may have opted to rest Ramos – but the 29-year-old is his skipper, and this is El Clasico.

The fact Ramos' regular defensive partner Raphael Varane is out with a calf problem increases the likelihood that he will be patched up and tasked with shackling 'MSN'.

Ramos remains one of the world's finest defenders – a man Suarez described last year as his toughest opponent. But Camp Nou is no place for an opposing player to work his way back to full fitness.

If Ramos is still struggling and 'MSN' are in the mood, it will be a long, hard evening for Madrid and their captain.

'Gracies Johan'

The man widely seen as the most influential figure in Barcelona's history passed away last week.

El Clasico will mark Barca's first game since the death of Johan Cruyff, and the club will honour the Dutchman with a minute's silence - accompanied by a video of his greatest moments - before kick-off.

Barca's players will wear special commemorative shirts with the words 'Gracies Johan' - Catalan for 'Thank you, Johan' - printed on the front, while all four sides of Camp Nou will form a mosaic with the same message as the teams emerge from the tunnel.

It is sure to be highly emotional. But will Barca's players and fans be inspired by the occasion and determined to give a footballing icon a fitting send-off on the pitch, or will they understandably be a little flat?

The answer to that question could be key to Madrid's hopes of returning to the capital with a positive result.

Zidane the player thrived on Clasicos. What about Zidane the coach?

"For a player, El Clasico is the most beautiful game that exists in football. I'm happy now to live it as a coach," Zidane told his pre-match media conference on Friday.

Zidane - the strutting, balletic genius who won every trophy there is to win - loved playing at Camp Nou in the white of Madrid.

However, managing in a Clasico is an altogether different matter.

Just three months in to his senior managerial career, will the Frenchman be able to cope on the touchline of Europe's biggest stadium, with the floodlights glaring down on him and nearly 100,000 fervent Barca fans baying for Madrid blood?

Clasicos are often decided by fine margins. Sure, Lionel Messi or Cristiano Ronaldo might win the game on their own with a moment of magic, but tactics and input from the technical area can also make the difference.

The last five Madrid managers have lost their first Clasico. Zidane must not freeze on the biggest stage of all if he is to avoid becoming number six.