It sounds like the musical version of a classic anxiety nightmare. You walk out on stage, sit down at the piano for a concerto, and the orchestra starts playing a different piece than the one you were prepared to perform.

And yet a few weeks ago it was all very real to Tianxu An, who lived through the strange episode in the finals of one of the most prestigious competitions anywhere.

The Curtis Institute of Music student, just 20, was ready to play the Tchaikovsky Piano Concerto No. 1 at the International Tchaikovsky Competition in Moscow in June. When conductor Vasily Petrenko brought in the orchestra for its brief introduction, the sound that came at An was that of Rachmaninoff’s Rhapsody on a Theme of Paganini.

“I was really surprised,” said An. “The problem was I needed to react in that kind of fast way, so there was no time for me to begin with some kind of emotion.”

Rather, it was pure muscle memory, he said.

Now the Philadelphia-based pianist is getting ready to play the same Rachmaninoff piece with considerably more intentionality. He is soloist in the work Tuesday at the Mann Center with the Philadelphia Orchestra in an all-Rachmaninoff program led by conductor Elim Chan.

In Moscow, An was a full beat late in his entrance, but caught on fast and says he soon felt at ease in the Rachmaninoff, which he had prepared but did not expect to come until after the Tchaikovsky, in the final round of the competition. But initially he could be seen giving the conductor what appeared to be a slightly annoyed look.

In truth, he wasn’t annoyed at all, he says.

‘I decided to just keep going’

“My problem is that my feelings do not match with my facial expression, so my face looks annoyed, but actually I was not annoyed, but a little bit confused. It was like a question mark. I wanted to see the conductor, I wanted him to do something. Obviously he didn’t do anything. He just kept going, so I decided to just keep going.”

An was offered a chance for a redo, but he declined.

For one thing, “I felt my physical strength could not recover, that it would not be as good as the first time.” For another, he said, the audience and the jury would have higher expectations. “I would need to play better the second time than the first time. And also it is not good for me or for the competition, because music is a kind of art. It’s only one time, you cannot try to save it.”

Competition officials apologized for the mix-up, which they called a “gross error” on the part of an employee. In the end, An was awarded fourth place plus a special prize for “courage and self-possession.” (First prize went to French pianist Alexandre Kantorow.)

An — whose name is pronounced chian-shoe an — says he felt he had already won by simply being there. He is not a seasoned competition pianist, he said, and to make it into the famed Tchaikovsky Competition at all was an honor.

Van Cliburn made a splash there as the gold-prize winner at the first competition in 1958, and the event has been a passage to visibility ever since for many major pianists — Vladimir Ashkenazy, Philadelphian Susan Starr, Barry Douglas, and Andras Schiff among them.

“I really feel thankful for this competition," says An, “because for me to be in the round of the 25 people going to Moscow, that for me has already been luck or fortune, so my feeling is I just wanted to play for the audience, for the jury. I should be thankful they gave me three times, including the first and second rounds, to show my talent. They gave me the chance to play, and that is a big hall.”

From Moscow to the Mann

It was another competition that brings An to the Mann stage this week. After starting studies at Curtis with piano professor Meng-Chieh Liu in 2015, he was the 2018 winner in the senior division of the Philadelphia Orchestra’s Albert M. Greenfield Student Competition, for which he prepared the formidable Brahms Piano Concerto No. 2 and Mozart’s sunny-to-serene K. 467 Piano Concerto in C Major.

Competitions, he says, can challenge him to “play better and in a more complete way. I think the nature of human beings is we tend to relax, we don’t want to challenge ourselves. Competition is a way to objectively push, to force us to play better.”

How does he feel he did in the Tchaikovsky Competition, all things considered?

“I think with the musical ideas, it was fine, because I think I projected and played the emotions I wanted. That’s the good part. But for maybe the bad part, because the order was switched, my physical strength was not equally distributed. Because maybe I put too much strength in the first piece.”

There was only a brief break in between pieces (as there was for all the pianists), he said, “so when I came back to play the Tchaikovsky, by the third movement I felt my strength was not enough.”

He had chosen the Rachmaninoff to go with the Tchaikovsky, he says, because to have put a full

Rachmaninoff concerto with the Tchaikovsky would have placed two big pieces alongside each other. “The Paganini variations is lighter,” he said, plus it allows both “virtuosity and intensity.”

He is particularly taken with the famous 18th variation, which he says to him seems to invoke the idea of memory, like an older person thinking back on youth.

“I think this piece fits my character,” he says.

Only next time, at the Mann, when the conductor begins with a gust of four 16th notes, both the character and piece may come across differently. For one thing, the pianist will be expecting it.

AT THE MANN

Upcoming Philadelphia Orchestra shows

The Philadelphia Orchestra performs an all-Rachmaninoff program Tuesday at 8 p.m. at the Mann Center, 52nd and Parkside Ave.

The orchestra’s Mann concerts continue July 24 in a space-themed program to celebrate the 50th anniversary of the Apollo 11 moon voyage; and July 26 playing live to screen in “Star Wars: The Empire Strikes Back.”

Tickets: $20-$95.