While studying solo performance in the master’s degree program at Tokyo University of the Arts, she joined Theater Erfurt in Germany for four months as a trainee. During that time, Fujii made her opera debut, performing Sandman/Dew fairy in Humperdinck’s "Hänsel und Gretel" and also held a solo art song concert in the same theatre. The following year, she underwent additional training at the Leipzig Opera.

During her stay in Europe, Rena Fujii appeared as the Queen of the Night in Mozart’s The Magic Flute at the State Theater of Košice in Slovakia and sang the role of Rosina in Rossini’s The Barber of Seville at the Baltic Opera in Gdańsk, Poland. At a summer music festival held in Łomża, Poland, she appeared as Rosina and as Gilda in Verdi’s Riogoletto.

Fujii has also appeared in many concerts, singing with the Cappella Istropolintana chamber orchestra, Polish National Radio Symphony Orchestra, Podlasie Opera and Philharmonic Symphony Orchestra, and Tokyo New City Orchestra, among others.

Her many accomplishments include : 1st place at the Mikulaš Schneider – Trnavský International Vocal Competition ; 1st place in the opera categoty and 3rd place in the song category at the International Antonín Dvořák Singing Competition ; 3rd place and honoravle mentins in three categories at the Ada Sari International Vocal Artistry Competiton ; the Olga Warla–Kolo Prize ath International Has Gabor Belvedere Singing Competition ; 1st place and the R.Strauss Prize at the Yuai German Lied Competition in 2012 ; 1st place at the Cercle Deux Colonnes ; 2nd place and the Iwatani Award (Audience Award) at the 83rd Music Competition of Japan ; and winner at the 7th Shizuoka Internatinal Opera Competition.

After returning to Japan, she appeared in concerts and Operas : International educational Pacific Music Festival Sapporo ; Korea-China-Japan Opera Gala Concert in Daegu ; Zerlina in "Don Giovanni" with Michiyoshi Inoue, the Jose Carreras Special Concert with Yutaka Sado. Since 2015 Fujii has appeared frequently in Seiji Ozawa Matsumoto Festival and Ozawa Seiji Music Academy Opera Project as Shepherdess and Owl in "L'enfant et les sortilèges" , Micaela in "Carmen", Lauretta in "Gianni Schicchi", Adele in "Die Fledermaus".

Her recent acclaimed work is B to C (from Baroque to Contemporary music) Recital in Tokyo Opera City. With the theme of "Muse for Artists", she performed an ambitious program mainly of German and French songs. In the commissioned work "KANNAGI" composed by Chikako Yamanaka, Fujii herself wrote the lyrics for the scene of the opening of the Amano-iwato (heavenly rock cave) in KOJIKI (Records of Ancient Matters), which was confessed by Ame-no-Uzume, the oldest performing arts god in Japan.

Rena Fujii actively researches art songs and her repertoire covers many languages, including German, French, and Czech. She has studied under Machiko Sakurai, Tasuku Naono, Akihiko Mori, Eva Blahová, Regina Wener–Dietrich, Walter Moore, Susan Manoff, and Isabel Garcisanz.