Real Madrid against Barcelona on Saturday will go some way to deciding the destination of this season's La Liga trophy, but it could go much further in defining the Santiago Bernabeu career of Cristiano Ronaldo.
Madrid versus Barca, BBC against MSN, Ronaldo against Lionel Messi – the stakes are always high in El Clasico. This particular battle, though, comes at a time when the Portugal captain's blue-chip stock is starting to fall.
Fewer than three months from his 31st birthday, the forward is under pressure to deliver after producing what has been, by his standards, a poor start to 2015-16 and his long-term future in Madrid is starting to come under real scrutiny.
Ronaldo is under contract until 2018, but his own cryptic comments over his future have done little to dampen speculation that has linked him with former club Manchester United and big-spending Ligue 1 giants Paris Saint-Germain.
That was followed by him whispering into the ear of PSG boss Laurent Blanc on the touchline during the recent Champions League match between the two sides, a moment that may have been entirely innocent, although it resulted in inevitable scrutiny.
After expressing his disappointment at the sacking of Carlo Ancelotti as head coach in the close-season, speculation persists that he is unhappy with the tactical approach and training methods of successor Rafael Benitez, another unwelcome storyline for Madrid.
While he struggled on the pitch, Ronaldo recently took part in a lengthy promotional tour to market his film documentary looking back at his magnificent accomplishments to date. Now is the time he must remind those who matter - such as president Florentino Perez - that he can still deliver in the biggest of matches and remain at the top in the years ahead.
Madrid are already three points off the pace as they bid to improve on a record of one league title in Ronaldo's six seasons in the Spanish capital. His supporters rightly claim he has done more than most to alter that statistic with a phenomenal goal record – but this season the numbers stack up against him.
The winner of the FIFA Ballon d'Or for the last two years has eight La Liga goals. That only puts him behind Neymar and Luis Suarez in the scoring charts, but five of those strikes came in one match against Espanyol, meaning he has only netted in four of his 11 appearances.
Now in his seventh campaign at Madrid, Ronaldo's chance conversion rate is the lowest it has ever been at this stage of the season, down at 11.9%. It has been a rapid decline for the attacker, as last season's record of taking 35.5% of his chances was the best start he had ever made to a league campaign as he registered 18 goals from his first 10 games.
Even though Benitez has repeatedly tinkered with his formation and personnel - a number of different players being trusted with the No.10 role while Gareth Bale has been out and Ronaldo occasionally playing as a central striker during Karim Benzema's injury absence – the Portuguese attacker has still had his shooting opportunities.
Ronaldo has had significantly more attempts on goal (78) than anyone else in Europe's top five leagues, with Caen's Andy Delort (63) and Bayern Munich star Robert Lewandowski (59) a distant second and third. His critics point to poor finishing and his backers suggest a combination of bad luck and inspired goalkeeping.
The shortage of goals has not been because he is creating more for his team-mates either, with only two assists to his name so far – a lower total than he has registered in all but one of the previous six campaigns.
El Clasico will likely see Ronaldo continue his personal duel with Messi, who looks increasingly likely to play following his knee injury.
The pair's records are almost inseparable since the Madrid star's arrival in the league in 2009. He has 233 goals from 211 Liga games and Messi has 235 from 212 over the same period, although the Argentine leads the way on assists (87 to 67) and is odds-on favourite to reclaim the Ballon d'Or for 2015 having helped Barca to an historic treble last season.
The only obstacle to Messi's continued brilliance appears to be injury. For Ronaldo, there are other questions to answer. What is his long-term future? Is this the start of a long goodbye at Madrid? Can he still compete with Messi now he is the wrong side of 30?
Stealing the headlines on Messi's return from injury would answer one of them at least. The others would remain up in the air, at least in the short-term, but a match-winning performance would be a timely nudge in the ribs of those in the corridors of power at the Bernabeu that Ronaldo may still be worth fighting for.
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