BARCELONA, Spain (AP) -Even though the season has barely begun, Real Madrid's classic meeting with Barcelona on Sunday is a must-win game for the defending Spanish champion, which visits Camp Nou already trailing the Catalan club by eight points.
Barcelona is perfect through the first six rounds and Tito Vilanova's squad has a chance to mark the club's best league start ever with a victory over its fierce rival.
After its poor start, Madrid has won its last three games, including a 4-1 rout at Ajax in the Champions League on Wednesday.
Madrid's Cristiano Ronaldo is in tremendous form having scored back-to-back hat tricks this week to take his season tally in all competitions to 12 goals.
"Scoring three goals is fundamental for me, it boosts my confidence," said Ronaldo. "We are going to have a tough game at Camp Nou, but we believe in our chances and are sure we will get a good result."
The Portugal forward has proven to be particularly effective against Barcelona, scoring in each of their last five meetings.
Barcelona will counter with Lionel Messi, who when not scoring has been the team's most creative playmaker at key moments this campaign.
Messi, who scored a Spanish league record 50 goals last season, has found the back of the net 10 times overall this season. And when defenses hemmed him in the last two games, he responded by setting up a pair of goals for his teammates to secure wins.
Madrid and Barcelona have already played twice this campaign in the Spanish Super Cup. Madrid lost 3-2 at Barcelona but escaped a larger deficit thanks to goalkeeper Victor Valdes' error to gift Angel Di Maria a goal. In the second leg, Madrid then used its best half hour of football against Barcelona in recent memory to establish a lead and held on against 10 men in the second half to claim the title. Madrid has won 88 "clasico" matches to Barcelona's 87.
"The game against Madrid is different from all the rest," said Messi, who has scored a club-high 15 times against Madrid. "The game on Sunday won't be about getting revenge (for the Super Cup), it's about extending our lead over a rival in the standings."
Barcelona's shaky defense was key in its demise in the Super Cup, and it will be put to the test on Sunday after Carles Puyol dislocated his left elbow during Tuesday's win at Benfica in the Champions League. Fellow Spain defender Gerard Pique will also likely miss the match due to a left foot sprain. That means Vilanova will again have to turn to Javier Mascherano and recent arrival Alex Song in the center of his defense.
Ronaldo said Barcelona will still be dangerous despite its thin back line.
"Barcelona is always complicated to play against," he said. "They have many options because they have a great squad and very talented players."
Also on Sunday, second-place Atletico Madrid hosts third-place Malaga in a game between the league's only other undefeated teams.
Malaga recorded an impressive 3-0 win at Anderlecht in the Champions League on Wednesday, while Atletico played Viktoria Plzen in the Europa League Thursday.
If Barcelona loses to Madrid, Atletico can become league leader with a sixth straight win.
Atletico striker Radamel Falcao, who leads the domestic competition with seven goals, is still questionable for the match.
On Saturday, last-place Espanyol plays at Valladolid with coach Mauricio Pochettino under increasing pressure to get a much-needed first win.
Pochettino, who is in his fifth season as manager of the Barcelona-based club, said that he doesn't think his "job is on the line" at Valladolid despite the fact that Espanyol has won just one point out of a possible 18.
Celta Vigo hosts Sevilla on Friday to open the seventh round.
Also on Saturday, it's: Rayo Vallecano vs. Deportivo La Coruna, Real Zaragoza vs. Getafe, and Real Betis vs. Real Sociedad.
Also on Sunday's, it's: Levante vs. Valencia, Mallorca vs. Granada, and Athletic Bilbao vs. Osasuna.