Five years ago, Gerard Mortier told Spanish composer Mauricio Sotelo that as much as he loved Lorca's surrealist play El Público, he did not quite understand it. He thought music might shed some light on its obscure meaning and suggested that an opera might give the play its utmost artistic form. After a laborious creative process, Tuesday's world première proved Mortier right and offered a colossal version of Lorca's masterpiece, while giving birth to one of the best operas of the new century.

Lorca's El Público can be defined as an abstract battlefield in which a theatre director confronts his love for another man under the shadow of his marriage with a woman. The stage becomes a parade of living metaphors that lead the director through a labyrinth of passion and sacrifice, a process of artistic exploration that ends in an immense void, only populated by the ghastly presence of the always staring public. It is fascinating how Andrés Ibáñez's libretto has used but a small part of the original text, respecting its tone and even enhancing its radical surrealism. Everything in the text invites music to fill in the gaps and to materialise the underlying rhythm of the play.

Mauricio Sotelo's score is a brilliantly implausible integration of spectral music and flamenco. The transition between musical languages is surprisingly smooth and flamenco music, a language mastered by Sotelo, coils seductively around the harmonic architecture, giving the impression of a musical continuum. He uses a chamber orchestra (the unmatched Klangforum Wien conducted by Pablo Heras Casado, who proved once again being among the best conductors of his generation), formed by 34 instruments and led by a Spanish guitar, whose sounds spread through the hall thanks to a system of loudspeakers that create a three-dimensional surround effect. Melody is constantly torn down by pure, primeval rhythm, marked by the hooves of the third horse (superbly played by bailaor Rubén Olmo) and impressive percussionist Agustín Diassera. Rather than an opera, Mauricio Sotelo has written the score of an ancient ritual, lost in the dreamy mists of surrealism but redolent of something older and darker.

To depict the complex set of transforming characters required by the libretto, Sotelo has chosen a classical vocal structure with carefully written lines that provide enough chromatic contrast. The title roles are played by two baritones. José Antonio López, light and lyric, gave a superb rendition as the Director, surmounting his vocal possibilities, with intense, tragic phrasing and convincing acting. Thomas Tatzl, a darker voice, is still a young promise yet to reach full maturity, but did a good job as Gonzalo, with surprisingly clear Spanish diction.

The two horses, acting as a sort of coryphaeus that give lyrical expansion to the drama, are played by two extraordinary and complementary flamenco singers: Arcángel, with an poignantly lyric voice, and Jesús Méndez, darker and more throaty. Tenor Erin Caves was just correct as the Emperor/Magician, a very demanding role that represents brute force and raw masculinity. The role of Juliet has a full-fledged coloratura aria in the third scene (rightly interpreted by Isabella Gaudí), which nevertheless fails to fit in the rest of the musical discourse. Finally, Antonio Lozano was correct as the shepherd and the third man, but quite disappointing in the luring role of the dark horse in the third scene.

One of the reasons of the indisputable triumph of El Público was the stage work of an exceptional artistic crew that was able not only to respect the surrealism of the piece and the subtlety of the music, but also to thrive within this abstract code, creating a fascinating visual performance. Director Robert Castro designed a stylised body-language that helped remove any realistic reference from the action. Alexander Polzin's set design was incredibly simple but had a mesmerising iconic strength, offering some of the best moments of the night: the transparent mirrors in the fourth scene, a well-known but extraordinarily used effect; and the bare stage of the fifth scene. Wojciech Dziedzic's costumes delicately conveyed the essence of each character (the empty costumes of the third scene were amazing) and Urs Schönebaum's lighting contributed to a heartbreaking final scene.

Apart from being a great opera, El Público epitomises a way of understanding this art that seems now in decline, after the recent losses of some of their prophets. This "theatre under the sand", as Lorca called it, is based on agonistic tragedy: theatre as a constant and painful struggle to reveal what hides under the mask, tragedy as an initiation sacrifice that excites a whole process of self-discovery. El Público stands out as a symmetric opposition to last season's Les contes d'Hoffmann, staged by Christoph Marthaler, where art was the Romantic consolation of a weary soul; and develops the dionysian invitation to the altar of theatre that Warlikowski made a few years ago in Król Roger. But for all the agony and pain the costumes of the third scene caress their empty faces, finding nothing under the mask. Once again, joining a glorious thread of lost steps, the waste stage recreated the tragic mystery of opera.