* Brazilian scores in 4-1 success at home to Sociedad
* Costa double helps Atletico maintain perfect start (Adds Atletico-Osasuna result, details, quotes, byline)
By Iain Rogers
MADRID, Sept 24 (Reuters) - Barcelona extended their winning start to their title defence to six games when Neymar scored his debut La Liga goal and Lionel Messi netted his seventh of the season in an entertaining 4-1 victory at home to Real Sociedad on Tuesday.
While Barca fans were treated to some breathtaking skill at the Nou Camp on the 56th anniversary of the inauguration of Europe's biggest stadium, Atletico Madrid gave another indication they may be challenging for the title come the end of the campaign with their sixth win in six matches.
Diego Costa struck twice in a 2-1 win at home to Osasuna to match Messi on seven for the season as Atletico joined Barca on a maximum 18 points, five ahead of Real Madrid, who play at promoted Elche on Wednesday.
Sociedad, who like Barca, Real and Atletico are competing in this term's Champions League, made the livelier start as they set about chasing their first league win at the Nou Camp in more than 20 years.
Their last victory there in May 1991, a 3-1 success which came when Barca had already been crowned champions, was thanks to two goals from John Aldridge and one from Dalian Atkinson.
Haris Seferovic cracked a shot against the crossbar in the second minute of Tuesday's game but the home side were ahead three minutes later when Neymar, recruited from Santos in the close season, bundled over the line after some indecision in the Sociedad defence.
World Player of the Year Messi made it 2-0 with a diving header from the Brazilian's cross three minutes later and Sergio Busquets scored a rare goal to make it 3-0 in the 23rd minute.
The Barca players appeared determined to put on a show after failing to convince in some of their games under new coach Gerardo Martino and showcased some of the rapid passing that has characterised their play in recent years.
One move towards the end of the first half was like a kick-around on the beach with Xavi, Andres Iniesta, Messi and Neymar tearing the Sociedad defence apart before Messi miscued Neymar's centre and the danger passed.
They lost some of their intensity in the second half and Alberto de la Bella pulled one back for the visitors just after the hour.
Messi, who squandered several chances, set up substitute Marc Bartra to score from close range 13 minutes from time and snuff out any hopes of a Sociedad fightback.
"I am very happy with my first goal in La Liga but the most important thing was the victory," Neymar said in a pitchside interview with Spanish television broadcaster Canal Plus.
"We are playing well, each day better," added the 21-year-old, whose assist for Messi was his third for the Argentine in the league this season.
Martino has been alternately resting some of his key players in the early stages of the season and Messi, who almost always plays a full match, was replaced with 10 minutes left.
He looked less than pleased with the decision but later posted a message on his Facebook page explaining that although he doesn't like to be substituted it's best for the team.
Atletico coach Diego Simeone has transformed the team since taking over at the end of 2011 and their success this season has in large measure been down to Costa's excellent form.
The combative Brazilian opened the scoring in the 18th minute at the Calderon, sidefooting Juanfran's cross past Osasuna goalkeeper Andres Fernandez, and made it 2-0 seven minutes later with a powerful header from Koke's centre.
Oriol Riera stole in to volley past Thibaut Courtois shortly before halftime but Atletico comfortably held on to their lead to secure a morale-boosting win ahead of Saturday's derby at Real's Bernabeu stadium.
In Tuesday's other games, Malaga won 2-0 at home to promoted Almeria to climb to seventh on eight points and Levante drew 1-1 against visiting Real Valladolid. (Reporting by Iain Rogers, editing by Justin Palmer)