Before the Second World War most music lovers, if asked to name the world's greatest classical guitarist, would have picked Andrés Segovia – and been unable to nominate a runner-up. No other instrument of importance has for so long been identified with a single player, and rarely has a musician of such eminence emerged from such an unstructured musical background. When he was five, Segovia was left in the care of an uncle and aunt who, noticing his response to music, arranged violin lessons with a teacher whose classes were as sterile as his approach was draconian. They were soon terminated. 

An itinerant flamenco guitarist then caught the child's rapt attention and, noting it, offered to teach him. In a few weeks the young Segovia had learned everything – very little in fact – the man knew. When he was 10 he was enrolled at a school in Granada where one of his classmates took him to visit a luthier's workshop and, seeing Segovia's interest, bought him a guitar and accepted lessons from him as repayment. Despite the opposition of his uncle and aunt, who had grander ambitions for their ward, he persevered, of necessity becoming his own teacher. 

At first he was dismayed by the apparent smallness of the guitar's repertory and the poor quality of most of it – he would later find it was larger and better than he thought – until his researches uncovered the compositions and transcriptions of Francisco Tarrega (1852-1909) , the father of the guitar's revival after its late 19th-century decline. Segovia learned much from him through his srudents Miguel Llobet and Emilio Pujol, both of whom he came to know. Tarrega's transcriptions and the pianistic efforts of a young lady friend also introduced him to the music of Chopin, Schumann and others, but he soon realised that more would be needed if the guitar was to enjoy a truly comprehensive repertory. 

Nothing had been written earlier than the closing years of the 18th century – around the time the instrument evolved – so he filled the lacuna with arrangements of Bach (which Tarrega had already done to a limited extent) and music for the lute and vihuela (the guitar's immediate Spanish predecessor). 

But the repertoire also needed a future, so Segovia approached well-known composers, using his performing skills as an inducement. Manuel de Falla had already written what would be his only work for the guitar – Homenaje, pour le tombeau de Claude Debussy, dedicated to Miguel Llobet – in 1920, and it was not until 1926 that Segovia scored his first success: Federico Moreno Torroba responded with his Suite castellana. Their collaboration continued until Torroba's death in 1982, a pattern he repeated with many later composers. 

Segovia gave his first small recital in 1909, the year Tárrega and Albéniz died, but his official debut was in 1912. His international career then developed rapidly, further accelerating after the Second World War when the growth in air travel put him in touch with far-flung audiences; the growth of the record industry, too, helped establish his reputation. The first recordings were made for HMV (1927-39) and during and after the war he recorded prolifically for Decca-Brunswick. 

His performing career continued until his death, but in its last decade the inevitable decline of his technique became painfully apparent. 'I will retire when my public retires from me,' he said but they never did. Concert halls were always filled with enthusiasts paying grateful homage to his past work on behalf of the guitar. The fact that he was remembering past glories in slow motion seemed unimportant, it was enough just to be there and catch fleeting moments of great artistry. On the other hand, the young, who experienced only the dying embers and had not heard the earlier recordings, were tempted to believe he had been overrated and overtaken by progress – a debatable proposition, even then. 

Segovia's unconventional formative history is central to an understanding of the man and his music-making because, in many ways, he was unique among the great virtuosi of the early 20th century. Critics have not been slow to point out the stylistic infelicities in his interpretation of pre-Classical music, a fate shared by others of his generation, Kreisler, Heifetz, Casals et al. But the benefits of post-war research came too late for them to absorb it into their bloodstreams, even if they'd had the free time to do so. 

Segovia grew up in Spain where Romanticism retained its grip longer than in any other major European country; he was also a Romantic at heart with little formal musical education, and was minimally equipped for serious musicological study. What he brought to this music (indeed, to everything he played) was love, respect and, in the case of Bach, an almost religious awe. His playing of Bach, Froberger and De Visée in the early recordings testifies to this. The stylistic peccadilloes are easy to detect from where we sit now, but were rarely noted by contemporary critics who responded to the human warmth and sincerity of the messages. 

Romanticism, which he perceived even in the music of Sor and Aguado – their Studies were central to his technical development – was at the core of his musical being. Few musicians have approached the affectionate expressiveness of his interpretations of small piano pieces by 19th-century composers from Schubert to Brahms, or the passionate intensity (sometimes bordering on ecstasy) of his response to Granados and Albeniz. In the case of the later Romantics such as Torroba, Turina and the neo-classicists, Ponce, Castelnuovo-Tedesco among them, what we hear in the early recordings are touchstone interpretations that were presumably shaped by the collaboration of the composers with the grateful artist for whom the works were written. Music in overtly 20th-century idioms was absent from Segovia's repertoire; its explosive evolution built a generation gap he could not bridge because he was unable to hear in such music anything of the beauty he considered essential to true art. 

Was Segovia the greatest guitarist of the 20th century? Yes...and maybe no. Some of his contemporaries were arguably his technical and even his musical equals. The earliest recordings by Llobet and Anido, and those made by Segovia before 1935, play back at pitches ranging from a semitone to a whole tone above concert pitch (and at correspondingly higher speeds), but that's purely a matter of old-versus-new equipment. 

Segovia's peers may have had the musical ability to trigger the revival of the guitar in the 20th century, but they lacked his burning ambition to spread the instrument's gospel throughout the world, and maybe even his physical stamina. While they were content to pursue their own comparatively modest careers, he was not, and therein lies the difference. A renowned harpsichordist, referring to Wanda Landowska, once told me she 'was a genius, but of course we don't do it that way now'. The same might be said of Segovia, but for whose work the guitar's 20th-century renaissance would almost certainly never have occurred.

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