Barcelona dropped two costly points in a 1-1 draw at Athletic Bilbao on Saturday to fall a point behind new leaders Real Madrid ahead of next Sunday's eagerly-anticipated 'El Clasico' at Camp Nou.
Brazilian Dani Alves put Barca ahead early in the second half but substitute Gaizka Toquero, only on the pitch a few minutes, equalised soon after as Barca surrendered top spot to Real.
"It is a bad result when you consider the opportunities we had," said Alves.
"We now have a tough Champions League game Inter and then Real. We just have to be positive and believe in ourselves."
Real produced a nervy 1-0 home win over Racing Santander on Saturday thanks to a Gonzalo Higuain strike and supplant their rivals at the summit.
"It is always complicated after a two-week break and coming off the back of the Kings Cup loss," explained Real coach Manuel Pellegrini.
"When you get towards the end of the match with only a goal cushion it is always risky but we had several chances and also hit the post twice."
Losing top spot now is an untimely blow for Barca who have two huge home games next week, beginning with a crucial Champions League match against Inter Milan on Tuesday and then the Clasico against Real at Camp Nou.
The media spotlight was on Barca forward Thierry Henry after his controversial handball which helped France eliminate the Republic of Ireland in the World Cup play-off.
However, coach Pep Guardiola took the forward out of the firing line starting him on the substitutes' bench, only introducing him in the 84th minute
Barca opted for a small front three of Lionel Messi, Andres Iniesta and Pedro with Zlatan Ibrahimovic out with a thigh injury which also makes him a doubt for the Inter game on Tuesday.
Messi should have scored in the 13th minute after being put clean through but couldn't get the ball out of his feet and his shot was blocked.
Messi wreaked havoc again when he rounded Iraizoz to set up a chance for Xavi who could not get a good enough contact to guide the ball in.
Bilbao had been second best but with seconds left in the first half Javi Martinez should have put the hosts in front but mistimed his header to miss the target and a sea of Athletic players held their heads in disappointment.
Barca got an early breakthrough in the second half with Xavi feeding a delightful pass to rampaging right back Alves who finished like a striker on 53 minutes.
However Toquero, only on the pitch five minutes, raced onto a headed flick to equalise nine minutes later.
Holders Barca now must focus on Inter in a must-win game with their title defence in the balance.
Real host FC Zurich on Wednesday needing a win with AC Milan on the same points and Marseille just a point behind.
Cristiano Ronaldo has missed ten consecutive games due to his troublesome ankle but could return ahead of the Barca clash.
Without Ronaldo Real have struggled in front of goal at times but got a helping hand from Racing goalkeeper Tono who foolishly pushed Kaka's cross into the danger zone and Higuain had an easy tap in on 22 minutes.
Sevilla kept up their own title charge with a 2-1 win at promoted Tenerife to stay three points behind leaders Real.
Argentine winger Diego Perotti and Brazilian Renato were on target as Sevilla kept pace with Barcelona and Real.
Manolo Jimenez's side now travel to Unirea Urziceni in the Champions League on Tuesday with qualification for the knockout stage secure.
Elsewhere Deportivo La Coruna moved up to fourth with a dramatic 2-1 home win over Atletico Madrid.
Mexican Andres Guardado converted a controversial penalty five minutes into stoppage time to down Atletico who stay in the bottom three with coach Quique Sanchez Flores still waiting for his first league win in charge.
"We weren't expecting that ending but things like that can happen in football," said Flores. "It is not going to be easy to bounce back in the league."