At the Verbier Festival, high in the Swiss Alps, a Ukrainian masterclass student is preparing to sing to Thomas Quasthoff. She rubs her palms nervously on her sleeves. “What’s the matter with your hands?” the forthright professor booms.

She admits to being scared. “But why?” he ripostes, in mock affront. “We never met before and I’m very nice !” She is persuaded to laugh – and soon her singing of Schubert is moving the audience to tears.

Quasthoff listens, acutely attuned to every nuance. Quick to correct her when necessary, he is also lavish with his praise. Quasthoff was born with phocomelia – severe birth defects caused by thalidomide, the morning sickness drug used in the 50s and 60s.

But that has not prevented the German singer from ­enjoying one of the most distinguished careers of any bass-baritone of his time. He is now 64 and this year marks the 50th anniversary of his first solo performance. In the interim he has sung everything from Bach to Wagner to Penderecki and has been awarded three Grammys, the Herbert von Karajan Music Prize, the Gold Medal of the Royal Philharmonic Society and Germany’s Order of Merit.

His voice is like an espresso martini: dark-hued, generous, yet bittersweet in expression, with a tremendously powerful kick. No wonder, then, that his 2012 announcement of retirement from public performance came as a shock to his fans. Talking to me after the masterclass, he explains.

“My brother died,” he says, upfront as ever. �.