Lionel Messi at double as Barcelona make record start
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Gerard Pique put the Catalan side ahead after 22 minutes followed quickly by Messi's first, which came off defender Fernando Amorebieta on the line, before Adriano Correia hit the third on the point of half-time.
Cesc Fabregas then added to the deluge of goals and Ibai Gomez got a consolation before Messi struck again.
Barca have now put together 13 wins and a draw from their first 14 matches to beat the previous best start in the Spanish top flight by Real Madrid in the 1991-92 season.
Messi also is now just one goal off Gerd Mueller's record of 85 goals in a calendar year for Bayern Munich and Germany in 1972.
Barcelona's injury problems at the back have eased so that coach Tito Vilanova was able to rest Carles Puyol with Javier Mascherano playing alongside Pique, but Dani Alves was missing and so Adriano came in at right back.
The home side began at a high tempo but while they pushed Athletic back the visitors kept faith with the coach Marcelo Bielsa's attacking philosophy as they looked to bring the ball out from defence.
Barca had their first sight of goal with a low drive from Andres Iniesta after 12 minutes which keeper Gorka Iraizoz saved comfortably.
They got their breakthrough after a spell of pressure.
Athletic failed to clear their lines from a corner and although Iraizoz repelled Fabregas' shot, Pique pounced on the rebound to knock it home.
Minutes later and Barca doubled their lead due in part to slack defending which has plagued Athletic's season and one of the reasons why they appear far short of the team which reached last season's Europa League and King's Cup finals.
Fernando Amorebieta failed to keep tight to Messi who through on goal slipped the ball past the keeper and a despairing dive from the defender only served to help the ball over the line.
The Barca players were queuing up to score the third before the break with Fabregas having a shot saved and Messi hitting the post before Adriano burst in from the right and fired a Fabregas pass into the roof of the net.
There was no let up after the restart and Iraizoz did well to block Messi but could do nothing to stop Fabregas who fired home an Iniesta pass inside the area.
Barca were guilty of easing off as Gomez pulled a goal back but Messi had the final say with his second.
Meanwhile, Mauricio Pellegrino was sacked as manager of Valencia following his side's 5-2 defeat at home to Real Sociedad.
The game swung on the first half dismissal of Jonas Goncalves with Valencia ahead through Roberto Soldado.
Alberto De la Bella equalised before the break and further goals came from Mikel Gonzalez, Diego Ifran, Imanol Agirretxe and Carlos Vela from the penalty spot while Soldado also got a second.
Earlier an Alberto Lopo header saw fourth-placed Malaga lose 1-0 away to Getafe.