Barcelona failed to carry on from where they left off in 2009, being held to a home draw 1-1 by lively Villarreal on Saturday, dpa reported.
The disappointing draw leaves Barca just three points ahead of second- place Real Madrid, who will go top on goal difference if they win Sunday at Osasuna.
Barca have 40 points from 16 games, while Real have 37 from 15.
It was the first home league match that Barca have failed to win this season. Villarreal have not lost in the Camp Nou since 2004.
"Do not think that we have started to relax after winning everything last year. We will not relax at all," Barca right-back Dani Alves was quick to say.
"What happened tonight is that Villarreal are a good team, who made things very difficult for us."
The game was preceded by a parade of the six trophies that Barca won in 2009, the best year in their 112-year history.
New idol Pedro gave Barca the lead in the eighth minute after a powerful shot from Thierry Henry hit the crossbar. Despite this bright start, Barca never looked comfortable against opponents who hassled and chased for every ball.
Villarreal midfielder David Fuster three times came close to levelling for the visitors. He finally did so in the 52nd minute by blasting home a cross shot from Cani.
Guiseppe Rossi came close to winning the match at the end for Villarreal, who move up to ninth.
Earlier Saturday, Valencia moved up to third by beating defensive Espanyol 1-0 thanks to an injury-time goal from supersub Nikola Zigic.
Zigic, who has hardly started a match since joining in 2007 from Racing Santander, headed in a perfect late centre from winger Joaquin four minutes into stoppage time.
It was no more than Valencia deserved, because they had dominated from start to finish. Spain striker David Villa came close to breaking the deadlock on five occasions and hit the post with a near- post header.
"It was an important goal," said Zigic.
He then sent a message to coach Unai Emery, saying: "I hope I can play a little bit more after this."
"Los Ches" are now third with 32 points from 16 games. They are two points above Sevilla, who are at crisis club Atletico Madrid in Saturday's late match.