His other major debuts occurred in the following venues and years: Buenos Aires (1902), London (1903), Milan (1904), Lisbon (1907), the Paris Opéra (1911) and the Theatro Municipal (São Paulo) (1911).

He reached the New York Metropolitan Opera relatively late in his career, in 1922, as Figaro in The Barber of Seville, having enlisted in the Italian army during World War I.

In the later years of his life, he maintained friendships with a variety of singers, notably with baritones Gino Bechi, Carlo Tagliabue and tenors Giacomo Lauri-Volpi and Giuseppe di Stefano.

[10] Ruffo's repertoire included most of the major baritone roles in French and Italian opera, including among others Rigoletto, Di Luna, Amonasro, Germont, Tonio, Rossini's Figaro, Valentin, Iago, Carlo (in both Ernani and La forza del destino), Nabucco, Vasco, Don Giovanni, Barnaba, Scarpia, Marcello, and Renato in Un ballo in maschera.

He was also renowned for his interpretations of several baritone parts in operas that are largely forgotten today, namely, the title roles in Ambroise Thomas's Hamlet and Franchetti's Cristoforo Colombo plus Cascart in Leoncavallo's Zazà and Neri in Giordano's La cena delle beffe.

Like his tenor contemporary Enrico Caruso, Ruffo was said to embody a new style of singing in which power, declamatory force and a rich, chesty tone eclipsed the previous generation's emphasis on vocal grace, flexibility and technical finesse.

However, according to modern-day critics like John Steane[11] and Michael Scott,[12] the difference between the two great baritones was not quite as clear cut as some have suggested in the past, because both Battistini and Ruffo displayed exceptional vocal agility and control plus the ability to sustain a long legato line.

Writing in the Gramophone magazine in 1928, the often acerbic British critic and future record producer, Walter Legge, lauded Ruffo's singing, recalling a recital that he had heard the baritone give six years earlier in London.

But more: Ruffo's infinite subtlety, variety of tone-colour, interpretive insight and sincerity, his magnificent control, stupendous breathing powers, and impeccable phrasing stamped him as a genius."

Surprisingly, they sang together infrequently and made only one commercially issued recording: an electrifying 1914 performance of the Oath Duet from Giuseppe Verdi's Otello.