In 2002, he made his debut at Polish National Opera as Gremin in Eugene Onegin, later appearing also in Die Zauberflöte (Sarastro), Lucia di Lammermoor (Raimondo), Nabucco (Zaccaria), Rigoletto (Sparafucile), Turandot (Timur), and The Haunted Manor (Zbigniew).

[3] The same year he sang the part of Tibault d’Arc in The Maid of Orleans in Concertgebouw in Amsterdam and Il Commendatore in Don Giovanni at Théâtre Royal de la Monnaie in Brussels.

Maazel also conducted operas in which Siwek sang bass parts – Aida in Sao Paulo (Ramfis) and Luisa Miller in Valencia (Wurm).

He sang Galitsky at the season opening performance of Prince Igor at Staatsoper in Hamburg and Commandore at Deutsche Oper in Berlin during the Festival Castell de Peralada.

In 2017, Siwek sang Heinrich in Lohengrin at Paris Opera,[9] Fiesco in Simon Boccanegra in Concertgebouw,[10] as well as Galitsky in Prince Igor[11] and Tsar Ivan the Terrible in The Maid of Pskov[12] at Bolshoi Theatre in Moscow.

In the next year, together with Placido Domingo, Siwek was one of the performers at Verdi Gala in Paris Philharmonic and sang Zaccaria in Nabucco in Bayerische Staatsoper in Munich.

n the 2020-21 season, during the pandemic, he sang in Poland among others Philip in the concert performance of Don Carlos at the Grand Theatre in Łódź (together with Aleksandra Kurzak, Roberto Alagna and Andrzej Dobber) and at the Arena di Verona Festival he created Zaccaria in Nabucco and Ramfis in Aida.

He created King Mark in Tristan and Isolde in Bari, Fiesca in Simone Boccanegra in Palermo and in Bologna he appeared in a concert performance of Tchaikovsky's Jolanta as René.

2022-23 season: Munich and Berlin - as the Grand Inquisitor, Vodnik in Dvořák's Rusalka at the Semperoper and in ROH in London; Sarastro in Die zauberflöte in Teatro Colon, Father Guardian in La forza del destino in Bologna.

His repertoire includes numerous parts in Verdi’s operas: Philip II and Grand Inquisitor (Don Carlo), Attila (Attila), Zaccaria (Nabucco), Wurm (Luisa Miller), Il Padre Guardiano (La forza del destino), Silva (Ernani), Ferrando (Il trovatore), Fiesco (Simon Boccanegra), and Massimiliano (I masnadieri); Wagner’s operas: Daland (Der fliegende Holländer), Heinrich (Lohengrin), King Marke (Tristan und Isolde) and Russian operas: Boris Godunov (Boris Godunov), Galitsky and Konchak in Prince Igor, and Tsar Ivan the Terrible in The Maid of Pskov.

He has sung with many acclaimed opera singers (e.g. Placido Domingo, José Cura, José van Dam, Juan Pons, Leo Nucci, Marcelo Alvarez) and conductors (e.g. Zubin Mehta, Lorin Maazel, Semyon Bychkov, Daniel Oren, Gianluigi Gelmetti, Krzysztof Penderecki, Helmuth Rilling, Alberto Zedda, Fabio Luisi, Walerij Giergijew, Kent Nagano, Roberto Abbado, Kiril Petrenko, and Marco Armiliato).

He took part in stagings by Franco Zeffirelli, Pierluigi Pier’Alli, Willy Decker, Jonathan Miller, David McVicar, Mariusz Treliński, Hugo de Ana, and Adrian Noble.

9 (Rome, Milan, Brussels, Taormin), Rossini’s Stabat Mater (Verona), Janáček’s Glagolitic Mass (Praha, Bonn, Bratislava, Brno, Linz).