Gabriela's free spirit

German audiences are not renowned for adaptability. Change is frowned upon and freedom is held to be inimical to the sacred ritual of classical performance. It takes a lot of courage to challenge a hall full of expectant faces but Gabriela Montero was not about to shrink, then or ever again, from being simply herself.

"Maybe I'm a rebel," she tells me, "but artists get asked three years in advance what we want to play in this hall or that festival. How do I know what I'll feel like playing in 2010? It makes no sense for me not to do improvisation when that feels right to me. Not to do it would be like giving only half of myself to an audience."

At 37, a single mother of two small girls, she has had more stops and starts to her career (and her love life) than most players have hotel breakfasts. But at the heart of her art is the certain knowledge that she can, when the spirit moves her, spontaneously invent a stream of music from deep within herself.

Improvisation is a defunct art. It's what virtuosi used to do before records and radio made them stick to a programme. Mozart freely improvised on his own tunes. Liszt would strike up an aria from a Wagner opera and decorate it. Ferruccio Busoni ruminated on a Bach theme, drawing wider and wider musical connotations.

After the Golden Age (the title of Kenneth Hamilton's new Oxford history of romantic pianism), that artistic licence was suppressed. Gabriela Montero, born in Caracas, Venezuela, sees herself as its modern liberator. She started young. "My first Christmas," she relates, "when I was seven months old, my grandmother bought a two-octave toy piano for my cousin who was three. I began to play the songs my mother sang to get me to sleep. She recorded me."

On her third birthday, Gabriela gave a recital made up entirely of improvisations. Formal lessons followed. At eight, she played a concerto with the Simon Bolivar Orchestra and won a stipend to study in Miami with a teacher she now dubs "the worst possible person I could have ended up with".

The 10 years she spent there almost put her off playing for life. "My teacher was mediocre in spirit and she wanted me to be like her. She managed to make me hate music, and therefore myself. I stopped playing for two years."

Back in Caracas: "I was not doing much, so I got married for the first time. I guess I had to do something." A friend got her to play the Brahms D minor concerto at two weeks' notice. Without pressure to succeed, all the old feelings for music returned. She soon applied for a scholarship to the Royal Academy in London, leaving the marriage behind. She met and married an English lawyer, the father of her elder daughter, Natalya, "but we were not meant for each other".

Entering the 1995 Chopin Competition in Warsaw, she spent two days in tears, unable to find a rehearsal piano. As she went on stage, "a heatwave went through my body. I played like I did not give a damn. It was mystical and out of myself. I knew that's what I had to connect to whenever I play".

She won third prize and a barrage of opportunities, "but then I lost my way again". She moved from London to Miami and back to Caracas. She lived in Montreal for three years - "in love with a Canadian" - then in Amsterdam, with a Dutch singer, and finally flew home to Caracas again, "because basically my heart was broken".

But in 2001 she met the Argentine virtuoso Martha Argerich, who changed her life. Argerich, hearing her improvise, urged her to take it seriously. "I was afraid to do it on stage, it felt improper. And Martha said to me: 'Gabriela, you can do this, and no one else can, why don't you do it?'" So she does.

We have reached the moment of truth.

Gabriela and I are sitting in a pianomaker's showroom where the grands are priced from £56,000 to £75,000. She gets a hard tone out of the first one she tries and shrieks: "Ikea!"

We find an instrument and she asks me for a theme. I suggest the opening of Mahler's fifth symphony. Being a pianist, she doesn't know it. I sing a few bars. She doodles around a bit. We have a go at Brahms's clarinet sonata. Nothing much. But then I try The Beatles' Yesterday and she's flying.

She plays the opening line once and repeats the first three notes, before veering into a baroque set that could have been written by Haydn's younger brother. "Can you do another style?" I ask. "Name one," says Gabriela. " Rachmaninov." And she's off again, now into dark Russian chromaticisms that Paul McCartney would die to have in one of his requiems. The beauty of her invention is in the structure: every part of what she plays is perfectly weighted against the rest.

"Piazzolla," I demand and we have Beatles, tango-style. "I could do this all night," she smiles. "I so love it. I come back from an airport, sit at the piano and clean my head with this." She has an album out on EMI this month and a free Close Up recital at the Royal Festival Hall next week. She also improvises on demand for visitors to her website.

She has bought a house near Boston and, though single, seems happy: "I feel that what I am doing now has some importance. I want to widen the parameters of the concert. I love the look on people's faces when it's unexpected."

www.gabrielamontero.com