MADRID: Barcelona's perfect record of 18 points from six Spanish League matches comes under threat on Saturday when the champions travel to Valencia in arguably their most difficult fixture so far.

Real Madrid have an easier task with a home game against Valladolid but must again do without Cristiano Ronaldo whose ankle injury flared up on international duty with Portugal.

Valencia are likely to be without star striker David Villa - who Barcelona tried to capture for a reported 40 million euros in the summer - due to a thigh injury but still have the firepower to hurt Barcelona in Spanish internationals Juan Mata and David Silva.

"We may have to bring in some back-up players to cover injuries but we can't start moaning about injuries to Villa, (Carlos) Marchena or anyone else," said veteran midfielder Ruben Baraja.

Barcelona equalled their best ever league start with six wins and hope to set a new club record by notching a seventh successive win at the Mestalla Stadium.

Madrid, who welcome Valladolid, fell to their first defeat of the season - a 2-1 loss at Sevilla - last time out and the sometimes dramatic local media decided to attribute the defeat to the absence of Ronaldo claiming Real suffered from 'Ronaldo dependency'.

Ronaldo has scored nine goals in seven games for Real but will again be missing after aggravating his ankle injury in a brief appearance for Portugal.

"I am not some kind of saviour but after the Sevilla defeat I wanted to help my teammates win the upcoming matches," explained Ronaldo. "I am sad because I was on a good run and getting injured again is a blow."

Sevilla outclassed Real to suggest they could disrupt the Madrid-Barca title monopoly, but need to build on that win at Deportivo La Coruna on Saturday.

Deportivo have had a great start to the season lying fourth in the standings after three straight wins while Sevilla are on a run of five consecutive victories.

It is a great time for Sevilla new boy Alvaro Negredo who helped defeat old club Real last time out, made his Spain debut against Armenia on Saturday and scored his first international goals against Bosnia-Herzegovina on Wednesday.

"I left Madrid because I wanted to play football and I am very happy with my choice to join Sevilla," said Negredo.

"It was a unique opportunity for me (to play for Spain) and I wanted to take advantage of the door that Vicente del Bosque (Spain national coach) opened for me to go to the World Cup."

Elsewhere only two teams have yet to win a league match with six games gone and they meet on Sunday when promoted Xerez host Villarreal.

Xerez were expected to struggle but Villarreal?s stuttering start has come as a surprise with coach Ernesto Valverde longing for his maiden win.

Three points from six games sees Villarreal languish second from bottom a point above basement boys Xerez.

"The international break has been good for us as it helps take our minds off our situation and prepare for our forthcoming matches," said Villarreal midfielder Robert Pires.

"We cannot let our heads go down because we have sufficient quality to get back on track soon."