When Barcelona beat Real Madrid 2-1 in October, the balance of power in Spain, despite major changes at both clubs during the summer, appeared to still very much rest with those in Catalonia.

The victory gave Barcelona a four-point lead on their rivals having won nine and drawn the other of their first 10 La Liga games of the season. Barcelona’s major summer arrival Neymar had scored one goal and created the other in the win at the Camp Nou. In contrast Gareth Bale, who Madrid had made the most expensive player of all time, was anonymous and hauled off after an hour following a start to his career in Spain beset by injuries.

Five months later, as the teams prepare to meet again on Sunday, it’s fair to say things have changed quite a bit. Having not lost since the last Clasico, it is now Madrid who go into the showdown with a four-point advantage on Barcelona and in the knowledge that a victory would surely knock their rivals out of the title race.

Madrid had plenty of teething problems under new coach Carlo Ancelotti, not least how to incorporate their array of attacking talent into a workable, balanced team. Results such as a 7-3 victory over Sevilla and a 3-2 win over Rayo Vallecano immediately following their defeat top Barcelona illustrated the problem. Yes, they could score for fun, but they were far too open and would be punished against high level opposition.

Then in January, Ancelotti, who has not always appeared the most tactically astute of coaches, came upon a solution. He switched to a 4-3-3, with the fit again Xabi Alonso as the midfield pivot. Alonso was supported by the unheralded star of Madrid’s season Luka Modric, while Angel di Maria has fairly seamlessly replicated his role for Argentina as a creative, energetic presence on the left of the midfield three.

Up front Cristiano Ronaldo has continued to be, well, Cristiano Ronaldo, while Karim Benzema has put his early-season struggles behind him and is in the best form of his Madrid career. Meanwhile, Bale has now settled nicely and is providing both individual moments of quality while being happy to adopt a team ethic.

At the same time as Madrid hit top form, Barcelona began to falter. Since the turn of the year they have lost three and drawn two of their 11 league matches, including a run of three defeats in just six games. Yet, while it is true to say that their results dipped, they far from came out of the blue. Questions have, often justifiably, been raised about Barcelona’s play since the start of the season.

Gerardo “Tata” Martino was handed something of a poisoned chalice when taking over from Tito Vilanova on the eve of the season. There has been dissention in the club’s hierarchy about the direction of the club on and off the pitch, while he inherited a squad that was aging together. The failure to keep hold of a player who should have been a focal point of the new Barcelona, Thiago Alcantara, was negligent in the extreme, as was the continued failure to sign a top class center-back.

Martino has moved away somewhat from the club’s strict tiki-taka philosophy which had been so successful under Pep Guardiola, leading to much debate in the hothouse of Catalonia. The defeat to Real Sociedad, when Alex Song calamitously operated in midfield, and subsequent loss to Real Valladolid both saw Barcelona almost unrecognizable from their glory years.

Since an ankle injury in January, Neymar has been unable to replicate the form at club level that he showed upon arrival. The controversy over his transfer has surely not helped and only added to the scrutiny on the Brazilian. With Lionel Messi now coming deeper as more of a creative presence, Barcelona have often been strangely insipid going forward.

That was until the past 10 days when the narrative heading into the Clasico has altered just slightly. After showing some attacking verve to finish off Manchester City to progress to the Champions League quarterfinals, Barcelona hammered Osasuna 7-0 with Messi firing a hat-trick.

And Barcelona will provide a real test for whether Madrid’s system can succeed against better opposition. Lacking a natural commanding presence without the ball in midfield due to Sami Khedira’s long-term injury, there is certainly a fear that they can be overrun. In the recent Madrid derby, Atletico created plenty of chances and were on top for a significant spell in a 2-2 draw. Barcelona will doubtless fancy their chances of passing their way around Madrid’s midfielders. The performance of Neymar could well be key, too, with his pace offering a valuable weapon to try and catch Madrid on the break.

Yet, as much as Barcelona can trouble Madrid, the same is very much true in reverse. Unable to control games quite as they once did, Barcelona’s neglected defense faces increased pressure. Manchester City had ample chances against them in the second leg of their tie while the pace and ability of Real Sociedad’s Antoine Griezmann and Carlos Vela tore Barcelona apart.

With Madrid’s “BBC” -- Bale, Benzema and CR7 -- going up against them, Madrid have to be favorites to get a precious victory.

Prediction: Real Madrid 2-1 Barcelona