Real Madrid find themselves eight points behind Barcelona after only four games of the Liga season following a 1-0 defeat to Sevilla at the Estadio Ramon Sanchez Pizjuan.
A goal after just two minutes from Piotr Trochowski as he exploited some poor marking from Angel Di Maria from a corner gave Sevilla an unexpected lead and Madrid, despite the depth of attacking talent the reigning champions possess, were unable to respond on a tense night in Seville.
Sergio Ramos and Luka Modric both hit the woodwork and Cristiano Ronaldo, still in a state of melancholy after declaring his unhappiness with life at the Bernabeu, appeared to be almost brought to tears when being denied by a superb challenge from Fernando Navarro as he threatened to score an equaliser.
A second defeat in four games leaves Madrid in a sorry state in La Liga, even at this embryonic stage of the season, with Ronaldo's self-proclaimed sadness, coupled with inadequacy on the pitch, putting Barcelona in firm control.
Madrid started the match knowing that a cavernous gap - by the standards set by La Liga's two giants at least - had been opened up by their great rivals from Catalunya, and perhaps it was playing on their minds in a disastrous start at the Estadio Ramon Sanchez Pizjuan.
Sevilla almost scored within the first minute when Iker Casillas - normally the ultra-reliable captain of the Spanish champions - spilled a curling effort from Jesus Navas and then desperately prevented Alvaro Negredo from tapping in the rebound.
However, Sevilla won a corner from the goalmouth melee and when Ivan Rakitic delivered the set-piece, Trochowski was allowed to run free by Angel Di Maria and the German midfielder swept the ball home from close range with a fine first-time finish on the half volley.
Madrid were rattled - as evidenced by Gonzalo Higuain's heated exchange with Fernando Navarro following a hard challenge on Alvaro Arbeloa which provoked a petulant kick-out from the Madrid defender - but almost restored their equilibrium when a thumping free kick from Ronaldo was only just palmed away by Andres Palop.
Ronaldo was rather subdued - perhaps a symptom of the mysterious malaise that has seen him declare he is unhappy - and flitted in and out of the first half, with one decent, dipping effort from long range being turned round the post by Palop.
Higuain was a more persistent threat, though his accuracy was off. The Argentinian striker met a fine cross from Marcelo with a poor header that flew wide before then hammering a shot off target following a neat spin which took him past Emir Spahic.
Sevilla's best avenue of attack was down the right wing, with full back Cicinho testing Casillas with a drilled effort from a tight angle and Navas doing likewise three minutes later despite Rakitic being in a far superior position in the centre and demanding a pass that never came.
Lacking their usual potency in the final third, Madrid were then lucky not to find themselves a man down heading into the break as Di Maria, already on a booking, reacted badly to a foul by Rakitic and kicked out at the Croatian. Mystifyingly the referee opted not to punish him.
It was no surprise to see the indisciplined Di Maria removed at the break and Ozil joined him as Mourinho brought on Luka Modric and Karim Benzema. Madrid's big summer signing from Tottenham needed little time to make his mark.
The Croatia international collected possession 25 yards from goal and unleashed a ferocious effort that appeared to be flying towards the top corner before Palop made a brilliant diving save. Higuain sent the follow-up wide, before later being denied by Palop following a fine through-ball from Modric, again.
In what was rapidly becoming an absorbing contest, Sevilla should have extended their lead when a poked pass from Navas set Negredo free in plenty of space. He had so much time to line up a shot from just outside the box but as Casillas braced himself for a save, his Spain colleague dragged the ball horribly across goal and wide.
It was now end-to-end, and Madrid's counterpunch came when Palop flapped at a free kick from the left and Benzema poked the ball back across goal, where Ramos was waiting. Two yards out, and at the home of his former employers, the defender somehow contrived to head the ball against the bar, much to his colleagues' frustration.
Madrid's continual inability to rein in Sevilla convinced Mourinho to make a rather reckless third substitution as midfielder Jose Maria Callejon came on for Arbeloa. However, the change failed to help open up Sevilla, who defended resolutely as the minutes wore away.
Ronaldo threatened to burst through on goal following a wonderful pass from Higuain late on but Navarro slid in with a perfectly timed challenge to keep Madrid at bay again. And when Ronaldo thumped a free kick over the bar deep into injury time, Madrid's fate was set.