Even with one of the world's most athletic opera singers at her side and plenty of practice, American mezzo-soprano Joyce DiDonato reckons the latest London production of Gioachino Rossini's La Donna Del Lago is the challenge of a lifetime.

DiDonato and Peruvian tenor Juan Diego Florez told Reuters at the Royal Opera House production's opening on Friday night that performing their technically demanding roles in the John Fulljames interpretation is almost like an elite sport.

"When you're on stage and you know you have to hit this incredibly difficult note, and there's an exhilarating silence in the audience and everybody is zeroed in on you, it's like you're running for gold in the Olympics," DiDonato said.

La Donna del Lago (Lady of the Lake) is based on an 1810 Walter Scott poem in which Didonato plays the ginger-haired Elena: the enchanting Lady of the Lake who finds herself at the center of - not one - but two love triangles.

Florez, who sings the role of her love-struck suitor King James V and is well known for his incredible vocal range, said the piece never gets easier.

"Everything about it is difficult," Florez said. "I have performed the part many times before but it is still a huge challenge."

Elena's father is fighting with the clans against the king's rule and has promised his daughter's hand in marriage to the brutish clan leader, Rodrigo.

But Elena is smitten with the young romantic hero Malcolm and at the same time has become the object of desire for King James - who disguises himself as a hunter to win her heart.

Florez, known in the opera world as a vocal acrobat, plays a domineering king whose steely veneer crumbles when he encounters the striking Elena, revealing his sensitive core.

The tenor showcases his vocal brilliance in a heart-wrenching aria in which the king declares his love.

He doesn't quite eclipse his showstopping "Ah! Mes Amis" from Donizetti's La Fille du Régiment, where he hits nine top C's, but certainly rivals it as a piece de resistance.

Although he is frequently cast in Rossini, Donizetti or Bellini operas, where rapid-fire strings of notes regularly loop up and down the musical staff, Florez said he is always looking for new challenges.

"I know what I can do and I know what I'm most comfortable with but I'm always looking for new roles," he said.