* Barca make it five wins out of five with 4-0 victory
* Atletico keep perfect record with 2-0 Valladolid win
* Misfiring Sociedad held at home by Malaga (Updates after Atletico game, adds quote)
By Mark Elkington
MADRID, Sept 21 (Reuters) - Barcelona maintained their perfect start to La Liga with a 4-0 victory at Rayo Vallecano on Saturday, a Pedro hat-trick and a penalty save by Victor Valdes helping them to win a game that was not a simple as it looked.
Spain striker Pedro took his tally for the season to five goals, while Cesc Fabregas netted the fourth, after Rayo had made life difficult for the visitors becoming the first team to edge them for possession in a match in five years.
The champions have 15 points from five games, and moved top ahead of Atletico Madrid on goal difference, after Diego Simeone's side also maintained their 100 percent record with a 2-0 win at Real Valladolid.
Fourth-placed Real Madrid welcome Getafe on Sunday when Villarreal, who also have 10 points in third, visit Celta Vigo.
Barcelona coach Gerardo Martino rotated his squad again with Dani Alves, Andres Iniesta and Alexis Sanchez starting on the bench, and his side were quickly on the back foot against a hard-running and aggressive Rayo.
Valdes was called into action after only three minutes as he saved a point-blank header from Alberto Perea, the compact pitch at Vallecas and a turbo-charged start from Rayo unsettling Barca.
An Alex Song interception was the starting point for the opener in the 33rd minute, as he set Lionel Messi on a jinking run past two defenders, and the Argentine forward laid the ball off for Pedro to fire low into the corner
Rayo's determination was undimmed, however, and minutes later Adriano bundled over Roberto Trashorras for a penalty.
Valdes saved a spot kick against Ajax Amsterdam in Barca's 4-0 Champions League win on Wednesday, and guessed right again as he denied Rayo captain Trashorras.
Rayo matched Barcelona in the first half but were caught out again as another interception saw Fabregas break down the left and cross low for Pedro to finish high into the net in the 47th.
Rayo began to run out of steam and Neymar almost bagged his first league goal for the club when he shot against the post, and shortly afterwards he selflessly squared for Pedro to tap home the third in the 72nd.
La Liga's leakiest defence then conceded a fourth seven minutes later after another interception on the edge of their area, and Fabregas calmly curled a shot inside the post.
"I liked the second half," Martino said. "As we did against Ajax we improved as the game progressed. The team has the ability to score goals and keeping a clean sheet always gives us a chance."
Atletico built on their flying start to the season and a midweek 3-1 Champions League win at home to Zenit St Petersburg with a typically effective display at Valladolid.
Both goals were set up by Spain midfielder Koke, who had a freekick deflected against the bar in the first half and floated in the cross for Raul Garcia to loop in a header in the 56th.
Koke then played a ball over the top for Diego Costa to net the decider on a breakaway in the 72nd, helping the Brazilian frontman to his fifth goal of the campaign.
Earlier, the demands of balancing domestic and European campaigns appeared to weigh heavy on Real Sociedad as they were held to a 0-0 draw at home to Malaga.
The San Sebastian-based club are back in the Champions League for the first time in a decade but suffered a shock 2-0 home defeat by Shakhtar Donetsk in their midweek opener.
After a slow start Sociedad gradually established themselves in the contest, with Imanol Agirretxe spurning a good opening and Jose Angel putting a freekick just over, but too often they were frustrated by Malaga's offside trap.
The last 15 minutes saw Malaga's Roque Santa Cruz twice go close to breaking the deadlock, and Sociedad's young manager Jagoba Arrasate will have been the most concerned at the end as his side finished a third consecutive game without scoring.
Sociedad stayed seventh with six points.
In Saturday's other game, 10-man Almeria again squandered a winning position as they lost a two-goal lead and were held to a 2-2 draw at home to Levante. (Reporting by Mark Elkington, editing by Stephen Wood and Ken Ferris)