Lionel Messi's 500th senior goal was not enough to prevent Barcelona from losing three La Liga games in a row for the first time since February 2003 as Valencia won 2-1 at Camp Nou.
The Catalans knew they could not afford to drop points again after previous defeats at the hands of Real Madrid and Real Sociedad - with Atletico Madrid moving level on points earlier on Sunday following their win over Granada - and created numerous chance to open the scoring in the opening stages of the game.
Luis Suarez, Messi and Neymar all failed to beat the excelling Diego Alves, though, as the Valencia goalkeeper pulled off a string of fine saves.
An own goal from Ivan Rakitic then saw Valencia take a shock lead, before Santi Mina doubled the visitors' lead with his fourth league goal of the season.
Messi eventually got his milestone goal after the break to end his barren run, but it was not enough to salvage a draw as Valencia held firm.
This weekend's results mean Barcelona are level on 76 points with Atletico, while third-placed Real Madrid have moved to within just one point as the title race hots up.
Barcelona dominated proceedings right from the start and created a number of chances to go a goal ahead.
Suarez volleyed over the crossbar in the fourth minute after being set up by Messi, before the Argentina international was denied from close range by Diego Alves just minutes later following some good work from the ex-Liverpool striker.
Neymar then failed to lift the ball over the Valencia goalkeeper in the 12th minute after being sent through one-on-one by a brilliant pass from Messi.
Suarez and Messi failed to convert further chances and the visitors made them pay in the 26th minute, Guilherme Siqueira's dangerous cross from the left clipping Rakitic's boot on its way past Claudio Bravo.
And Valencia doubled their lead just before the break through Mina. Dani Parejo set up the 20-year-old with a clever throughball to expose Sergi Roberto down the right and he beat Bravo with a composed finish.
Rakitic could have pulled one back early in the second half after an intelligent ball from Suarez, only for the midfielder to mishit his shot and fire wide.
Messi at last got his goal shortly after the hour mark to give Barcelona hope again, tapping home from close range after a low cross from Jordi Alba from the left.
Diego Alves showed his class again with 20 minutes left on the clock, keeping out Neymar's left-footed shot from inside the area.
The Valencia goalkeeper continued to frustrate Barcelona, pulling off a superb reflex to prevent Rakitic from making amends for his first-half own goal.
Neymar could have salvaged a draw with a late chance, but the Brazilian's shot bounced wide after taking a deflection, before Gerard Pique missed a sitter in the 90th minute as Valencia held out for a hard-fought win.