Madrid - Looking for its sixth win a row, Villarreal is the team to beat in Spain.
Villarreal plays at winless Levante on Sunday, while Barcelona, still without Lionel Messi is at Sevilla and Real Madrid faces a difficult game at city rival Atletico Madrid.
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Unbeaten and playing some entertaining football, Villarreal has 16 points from six games, one more than Barcelona and two more than Real Madrid.
The team from eastern Spain, whose best-ever result in the Spanish league was a second-place finish in 2008, has been relying on solid defense to keep its momentum going.
"It's been key in all matches we have played," Villarreal coach Marcelino Garcia Toral said. "We defended very well against Athletic Bilbao and Atletico Madrid, which are very powerful clubs, so it means that we can definitely defend well."
Villarreal has the fourth-best defense in the league with four goals conceded, only three more than Real Madrid. It hasn't conceded in the last two matches, including in the important win against title contender Atletico Madrid on Saturday at El Madrigal Stadium.
Levante, meanwhile, is one of two teams yet to win this season, along with Malaga. It has the second-worst defense with 12 goals conceded.
Here are some other things to look for this weekend in the Spanish league:
Cristiano Ronaldo surpassed Raul as Real Madrid's all-time leading scorer on Wednesday, netting his 323rd and 324th goals with the club in the 2-0 win against Malmo in the Champions League.
But Madrid is coming off a disappointing scoreless home draw against Malaga in the Spanish league, when Ronaldo and his teammates missed chance after chance and squandered important points at home.
The chance to rebound will come Sunday at the Vicente Calderon Stadium, against Atletico Madrid.
"We managers are eternally dissatisfied and we're always looking to cut out mistakes and improve things," Madrid coach Rafa Benitez said after the Malmo win. "I'm very pleased with the team, but it's my responsibility to try and improve things."
It will help that Benitez may be able to count on playmaker James Rodriguez, who has recovered from an injury that had kept him sidelined in recent matches.
It will be the second high-profile home game for Atletico Madrid, which already fell 2-1 to Barcelona this season. The team hasn't been showing its usual confidence while playing at home recently, and it showed on Wednesday in a 2-1 loss to Benfica in the Champions League.
Barcelona is dealing with more injuries than usual early in the season and the absence of important players has kept the defending champions away from the top of the standings.
After losing midfielder Rafinha for the season and having to deal with the absence of Lionel Messi for about two months because of knee injuries, Barcelona coach Luis Enrique will also have to find a way to set up his team without playmaker Andres Iniesta, who is nursing a muscle injury and won't play Saturday at Sevilla.
"We've lost some important players, but it's part of football," Barcelona defender Javier Mascherano said. "We have to keep going. We need to keep winning with the players we have."
Other players still injured include goalkeeper Claudio Bravo and defenders Adriano and Thomas Vermaelen.
Barcelona hasn't lost at the Sanchez Pizjuan Stadium in Sevilla since the 2006-07 season, when Dani Alves scored the winner while playing for the hosts. Barcelona has won five of the last eight games at the stadium.