Barcelona completed a wretched week when they surrendered the leadership of the Primera Liga to arch-rivals Real Madrid after they were held to a 1-1 draw at home to Real Betis on Sunday.
Barca, who saw their double dreams go up in smoke when they were given a 4-0 mauling by Bernd Schuster's Getafe in the King's Cup in midweek, looked to be heading for a nervous win after Ronaldinho gave them an early lead from the penalty spot.
But Betis dashed their hopes of returning to the top of the table when Brazilian striker Rafael Sobis took advantage of some shaky defending to drill home at the near post two minutes from time.
The result means that with four games left to play Real and Barca are level on 66 points but the nine-times European champions top the standings by virtue of their better head-to-head record.
It is the first time that Real have ended a round of matches at the top of the table since October 2005.
"There are still games left to play and we can't drop our heads now," said Barca coach Frank Rijkaard. "It is a shame we didn't win but we have to retain our pride. The league isn't decided yet."
Real went top on Saturday after staging a dramatic fightback to come from 3-1 down to snatch a 4-3 win at home to Espanyol thanks to a last-gasp strike from Argentine forward Gonzalo Higuain.
Sevilla are in third, just two points behind the leaders, after they warmed up for Wednesday's all-Spanish UEFA Cup final against Espanyol with a 2-1 win over Recreativo Huelva on Saturday.
Valencia tightened their grip on fourth spot, and are now only four points behind the leaders, after a classy 2-0 win at home to Real Zaragoza, the goals coming from defender Emiliano Moretti and talented forward David Silva.
Barca looked as though they might be able to put their humiliating Cup defeat behind them when they were awarded a penalty after Betis defender Melli brought down Deco as he burst into the area in the fifth minute.
Ronaldinho smashed the spot kick high into the net to take his tally for the season to 18 goals.
Barca dominated thereafter but despite spectacular efforts from Samuel Eto'o and Andres Iniesta they were unable to extend their lead.
Rijkaard opted to try to safeguard the result by replacing Ronaldinho and Lionel Messi after the break but his side never looked comfortable in defence and keeper Victor Valdes had to produce several saves to stop Betis from equalising.
Two minutes from time Betis midfielder Marcos Assuncao released Sobis into a gap on the right and the striker beat keeper Victor Valdes at the near post with a powerful low shot to the dismay of the Nou Camp faithful.
Earlier basement side Gimnastic Tarragona were left with one foot in the second division after a 3-1 defeat at home to fellow strugglers Real Sociedad.
Defeat means that the Catalans are now 10 points adrift of safety with four games left to play.
Celta Vigo also suffered a major blow to their chances of avoiding the drop when they slumped to a 2-1 defeat at home to Levante in another relegation struggle, their fourth defeat in a row under new coach Hristo Stoichkov.