American pianist Seymour Lipkin, a student of Rudolf Serkin recalled a performance of Beethoven’s Hammerklavier Sonata

Back in the 1970s I gave a recital at Curtis at Mr. Serkin’s invitation.  I was playing the Hammerklavier in those years.  Why, in my right mind … I should never have … but I did.  There, sitting in the front row, was Serkin.  Now, that was tantamount to suicide to walk out and play the Hammerklavier with Rudolf Serkin under your nose.  Somehow I got through it.  In all my previous performances of the sonata I’d played the opening with two hands, as almost every pianist in the world does.  I hadn’t wanted to make a mess of it at the very beginning.  Moments before going on stage I had a fit of conscience.  “My god, Serkin’s sitting there; if he sees me play that with two hands, he’ll never forgive me.  He won’t speak to me for the rest of my life.”  I had never even practiced it with one hand.  But, I knew, it had to be all or nothing.  Da-dum!  And for the only time in my life I hit it absolutely perfectly.  Every performance thereafter, when I attempted it with one hand, I made a god-awful mess. He was so proud of me afterwards.  “Wonderful, with one hand! Courageous of you! Wonderful.” “Thank you, Mr. Serkin, I do it all the time.”

Cited in: Lehmann, Stephen (2003) Rudolf Serkin: A Life.  New York: Oxford University Press, p. 211-212.