Victoria Pendleton says she is struggling on Strictly Come Dancing, because she has forgotten how to use her brain.
The Olympic gold medallist and former British track cyclist is competing against the likes of Kimberley Walsh, Denise Van Outen, Fern Britton, Jerry Hall, Johnny Ball, Sid Owen and Nicky Byrne in the BBC One show.
But she told the Radio Times that both her "body and brain" were struggling as she prepared for the first show in which the couples face judges Craig Revel Horwood, Darcey Bussell, Len Goodman and Bruno Tonioli for the first time.
She said: "Aside from my feet - which are still in agony - the physical exertion isn't very challenging, but the mental element - the counting, the timing, the remembering of the steps and every single movement - is really fatiguing.
"How can people remember to do that all at the same time?"
Victoria, who has two Olympic gold medals to her name, added: "I spent so many hours training as a cyclist that when I was on my bike I didn't need to use my brain any more. Everything just happened automatically, and all I had to concentrate on was giving 100 per cent.
"Even when I needed to be tactical during a race, I didn't plan ahead - it was just a matter of instinctively reacting to what was going on around me."
She said of her professional dance partner, Brendan Cole: "Brendan is teaching me the choreography in chunks and I keep finding myself getting to the end of one chunk and then going blank. My brain is just not used to thinking ahead and I'm having to relearn how to learn."