Think of German singers and it's heldentenors and the repertoire of their native country which comes immediately to mind. Apart from the great Fritz Wunderlich, it's unusual to have a teutonic singer who reaches much beyond Wagner, Mozart and Strauss, which makes the achievements of Jonas Kaufmann all the greater.

Perhaps the finest tenor to come out of Germany in recent decades, Kaufmann has an even wider repertoire than Wunderlich, excelling in Italian and French roles, and only recently tackling Wagner. Certainly, in London we've only seen him so far in Italian and French opera: La Rondine, La traviata, Tosca, Carmen and from next week the title role in Don Carlo. Hopefully, it won't be too long before Covent Garden sees his Lohengrin, which stunned critics and audiences in his home town of Munich earlier this year and is due to do the same in Bayreuth in 2011.

Don Carlo

In the meantime, his Covent Garden role debut as Don Carlo next week is in the first revival of Nicholas Hytner's production of the five act Italian version, new last June. Meeting Kaufmann in the Royal Opera's press office, I ask him about the relative strengths of the different versions. "Of course, it was written in French," he says, "and it fits perfectly in French. In Italian, you can see that it's sort of made to fit. It's pasted on at times."

I am treated to him singing a section of the "Io l'ho perduta" aria from the four act version, as an illustration of how the music has been made to work in Italian. He says he'd very much like to sing the role in French ("Maybe, not now because that could be very confusing, but yes one day"), having to date only sung the four act Italian version, in Zurich.

Nicholas Hytner

He's enjoying working with National Theatre director Hytner, who brings a classical, almost Shakespearian, dimension to the work. "That makes perfect sense," he says, "the audience can see details and can work out the relationships between people. It's all very clear. We've looked at the text in detail, with reference to Schiller's original." Kaufmann studied the classic play at school and is now benefitting from the director's rigorous textual analysis. "He's very willing to change things, too," he tells me. "Some directors don't accept change from first runs (the production opened last year with Rolando Villazón in the role) but Nicholas is very flexible and open."

For a tenor with so dark and baritonal a tone, his speaking voice is light and gentle. Flexibility is a word that crops up a lot when he talks about the qualities he brings to so wide-ranging a repertoire, which has seen him sing everything from Parsifal, Lohengrin and Florestan to Don Jose, Berlioz's Faust and the Puccini leads. "It's so important to keep what you do fresh, interesting and new. I want to do this job for forty years or more and the wider the repertoire, the easier it is to do that. If you do the same roles, or even the same composer, it's boring. And also dangerous. It means you're not challenging or controlling yourself. It becomes automatic and you start making mistakes."

He says his chosen path is hard work: "You're forever learning new roles but I love it." He demonstrates the qualities needed for different repertoire – portamento for Italian parts, elegant phrasing without portamento for Mozart, a strong, broadbrush approach for the heavier German roles – a mini-masterclass in vocal technique. "It is stressful trying to do everything," he admits, "It's so much easier to concentrate on doing one thing well but that's not what I want."

New Roles

Kaufmann has debuts as Werther in Paris in the New Year and Siegmund at the Met in 2011. I ask him about other potential new roles. Both Tristan and Otello are "at the end of a long list. In maybe five to 10 years." He expresses great interest in Peter Grimes, which would represent another widening, this time into the English repertoire.

He is very careful about the way his career unfolds ("Otello should come after a Trovatore and Ballo"), as much as an artist can be when he's having to schedule performances years ahead. "Ten years ago, I would never have expected to be singing Wagner roles," he says, "Now I've sung Parsifal, Walther (in concert) and Lohengrin and I'm very comfortable with those parts, so you can't always predict." He talks of the need to balance what he wants to do with what's right for his voice at the time: "You have to guess ahead which parts you're going to be ready for."

New recording

There have been few recordings to date. A CD titled Romantic Arias was a massive hit a couple of years ago and this week sees the UK release of his latest album (called Sehnsucht in Germany, it's untitled here). The previous one included French and Italian works but this time it's all-German, ranging from the big Lohengrin numbers to Fidelio's "Gott! Welch Dunkel hier!", Tamino's arias from Die Zauberflöte and lesser-known pieces from Schubert's Fierrabras and Alfonso and Estrella.

Going down the glamour route, despite Kaufmann's obvious strengths in that area, is not something he's interested in. He eschews "cross-over" ("why would you want to do that?") and enjoys a success that avoids overt celebrity. "I'm happy that I'm not so popular that I can't walk over to Covent Garden without being pointed at," he says, "I wouldn't want that." For a tenor, who could very easily have been lured down the path of photoshoots and easy money, he shows tremendous integrity. With great looks, a fabulous voice, a natural modesty and devotion to family (he has three children with his mezzo wife Margarete Joswig), it all adds up to an incredibly impressive profile.

Future London roles

Over the next few seasons, there's the thrilling prospect of Kaufmann's appearance at the Royal Opera in Berlioz's Les Troyens and a characteristically contrasting Adriana Lecouvreur, Cilea's lightweight romance.

In the meantime, his Don Carlo in this month's run promises to lift Nicholas Hytner's production to greater heights.

Don Carlo plays at the Royal Opera House on 15,18,21,23,27 September, 1 October 2009. Tickets are available on 020 7304 4000 or www.roh.org.uk. Jonas Kaufmann's new album (Decca) will be released in the UK on 14 September.

- Simon Thomas