I consider Franz Liszt to be the greatest pianist of all time. Not only did he sight read chopin's etudes perfectly, and in a manner which even chopin was jealous of, but he also sight read Grieg's violin concerto, both the piano accompaniment and the violin part. Here is the direct quote from Grieg:
That's strange. i wasn't aware that Grieg wrote a violin concerto. That's the first time ever I that I read about this work. Do you have a reference for this work , An opus number ? I can't find anything about it.
Maybe Grieg made concerto's out of his violin sonatas?
In 1868, Franz Liszt, who had not yet met Grieg, wrote a testimonial for him to the Norwegian Ministry of Education, which led to Grieg's obtaining a travel grant. The two men met in Rome in 1870. On Grieg's first visit, they went over Grieg's Violin Sonata No. 1, which pleased Liszt greatly. On his second visit, in April, Grieg brought with him the manuscript of his Piano Concerto, which Liszt proceeded to sightread (including the orchestral arrangement). Liszt's rendition greatly impressed his audience, although Grieg gently pointed out to him that he played the first movement too quickly. Liszt also gave Grieg some advice on orchestration (for example, to give the melody of the second theme in the first movement to a solo trumpet).