(Adds Messi's first name in para 12)
* Champions turn in woeful performance to lose 1-0
* Real can go three points clear with win on Sunday
* Atletico provisional leaders after victory at Celta
By Iain Rogers
MADRID, March 8 (Reuters) - Barcelona's bid for a fifth La Liga title in six years suffered a surprise setback when they slumped to a 1-0 defeat at lowly Real Valladolid on Saturday which opened the door for Real Madrid to pull away at the top.
It was one of Barca's worst performances of the season, particularly during an atrocious first half when Juventus cast-off Fausto Rossi scored for the home side, and raised serious doubts about their form ahead of Wednesday's Champions League last 16, second leg at home to Manchester City.
Real, who host Barca in 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.
Barca slipped to third and Atletico Madrid climbed into provisional first place thanks to a 2-0 win at Celta Vigo when former Barca forward David Villa struck twice in two minutes to improve his chances of making Spain's World Cup squad.
Real and Atletico each have 64 points, with the latter top due to a superior head-to-head record over their city neighbours, and Barca are on 63.
Valladolid climbed out of the relegation zone into 17th.
Most of the Barca players were in international action in midweek and many looked jaded and short of inspiration in a lethargic display at Valladolid's Jose Zorrilla stadium marked by uncharacteristic sloppiness.
"We failed to create going forward, in our passing and in our mobility and that's why we did not get the desired result," coach Gerardo Martino told a news conference.
"We are finding it hard to put together a complete game in every aspect of play, a perfect game," added the Argentine, who is in his first term in charge after taking over from the ailing Tito Vilanova.
"We are obliged to fight for the league until the end and that's what we will do as long as we have a chance.
"Right now we are not on top form and we have to get there to be able to take on a team like Real Madrid. We have 15 days to do it."
The first genuine chance of the match fell to Lionel Messi when he sprinted into space on the left in the 10th minute.
It was the kind of opening the four-times World Player of the Year normally converts with a minimum of fuss but his rising shot was straight at Diego Marino and the home goalkeeper palmed the ball to safety.
Valladolid grabbed the lead seven minutes later when Rossi was first to a loose ball in Barca's penalty area and the Italian, on loan from Serie A champions Juve, fired a first-time effort past Victor Valdes.
Messi and Fabregas again shot straight at Merino when well placed and Messi had another effort early in the second half before Neymar, who netted a hat-trick for Brazil against South Africa on Wednesday, fluffed Barca's best chance of the game.
The ball fell kindly for him on the left of the area but his wild shot flew over the crossbar into the stands, prompting derisive whistles from the home fans.
Martino brought Alexis Sanchez on for Fabregas and later switched to what was effectively a two-man defence of Javier Mascherano and Sergio Busquets but Valladolid held firm and came close to another goal on the breakaway late on.
Barca's latest setback followed last month's defeats at home to Valencia and at Real Sociedad and will give Manchester City hope they can overturn a 2-0 deficit at the Nou Camp.
"We cannot draw any positive conclusions, we just have to try and improve before Wednesday," Barca playmaker Xavi told reporters.
"It's a very painful defeat, a step backwards, but we have to turn things around before the Champions League."
Atletico drew 2-2 with Real at their Calderon stadium last weekend and stayed in the hunt for a first title since 1996 with an efficient performance in Vigo.
Villa, playing in place of suspended top scorer Diego Costa, punished a defensive error in the 62nd minute and conjured another clinical finish in the 64th when he swept the ball low into the corner from the edge of the area.
Spain's record scorer with 56 goals in 95 matches, Villa was left out of the world and European champions' latest squad but Saturday's brace could help convince national team manager Vicente del Bosque he deserves a place at the finals in Brazil.
Fifth-placed Villarreal's hopes of a return to the Champions League next season faded when they were beaten 2-0 at Granada.
Athletic Bilbao, in fourth, can increase their lead over Villarreal to nine points with a victory at Valencia on Sunday and sixth-placed Sociedad can also overtake them if they win at home to Rayo Vallecano on Monday.
Bottom side Real Betis improved their slim chances of avoiding relegation when they won 2-0 at home to fellow strugglers Getafe.
Betis have 18 points in 20th, five behind second-bottom side Rayo Vallecano, who play at Real Sociedad on Monday.
Valladolid's win over Barca took them out of the drop zone above Almeria, who host Andalusian rivals Sevilla on Sunday. (Editing by Ed Osmond)