Barcelona's bid for a fifth La Liga title in six years suffered a surprise setback when the champions crashed to a 1-0 defeat at lowly Real Valladolid on Saturday.
It was one of Barca's worst performances of the season, particularly during a woeful first half when Fausto Rossi scored for the home side, and they squandered a chance to pull two points clear of Real Madrid at the top.
Barca's players, most of whom were in international action midweek, looked jaded and short on inspiration and Lionel Messi and Neymar wasted a host of opportunities in a generally lethargic display.
Real, who host Barca for the "Clasico" in two weeks, can stretch their lead over their arch rivals to four points with a win at home to Levante on Sunday.
Atletico Madrid will climb above Barca and draw level on 64 points with Real if they beat Celta Vigo later on Saturday.