Five-time Olympic gold medallist Ian Thorpe says he is more likely to fail than succeed at London 2012.
The Australian, 29, makes a competitive return to swimming next month after five years away from the sport.
Speaking exclusively to BBC Sport, Thorpe said: "I'm more concerned about [being really successful] because that's what I'm not prepared for.
"I'm more comfortable knowing that, chances are, I'm going to fail at this. I've become comfortable with that."
After winning two golds at the Athens Olympics in 2004, Thorpe took a year away from the sport before announcing his retirement in November 2006 aged 24.
He is now preparing to return to competition with the Australian squad at the Singapore World Cup meet on 4-5 November, with further meets in Beijing and Tokyo to follow.
Thorpe faces a huge challenge if he is to return to the gold medal-winning days of the last decade.
American Michael Phelps has dominated the sport since 2004, winning 14 Olympic gold medals in that period, including a record eight in Beijing 2008.
Speaking to BBC Sport's Nick Hope, Thorpe said: "My goal at the start - and this will remain my goal through the Olympics - was to be able to swim faster than I used to.
"If at any stage I didn't think I could do that, I don't know if I'd continue."
Thorpe became the youngest-ever swimmer to represent Australia in 1997, when he was just 14 and he won two golds at the World Championships in Perth as a 15-year-old.
He also won three golds at the 2000 Olympics in Sydney and two in Athens four years later - but says his focus now is on achieving fast times.
"Medals are good but, for us in swimming, times are better because you have very little control over who wins," Thorpe added.
"The best way to win the nicer-coloured medals is to make sure your process is a lot better than everyone else's. Simple as that. I also don't think you should just limit yourself to trying to achieve this result, because this is what people perceive as being the ultimate.
"Push yourself beyond that and take that attitude of not just trying to win by one tenth of a second when you can win by two seconds. Take it that far. That's athletic accomplishment, not just what you get to wear around your neck."
Thorpe is training in Lugano, Switzerland under the guidance of Gennadi Touretski. The Russia-born coach helped compatriot Alex Popov to four Olympic golds between 1992 and 1996.
And Thorpe added: "The decision to come to Switzerland was based on the coaches here.
"I think Touretski is the best sprint freestyle coach in the world.
"Given the motivating setting that I'm in, I realised that training was going to become a whole lot easier."
But the Australian does admit he has left it late in terms of preparation.
"I'm more prepared to struggle through this whole thing and just enjoy it because I've never experienced that in my career," said Thorpe.
"I don't think anyone's done this before with the time frame that I have. Realistically, it's probably too short.
"For some reason, in my mind, this is helping me. It's making it harder so I'm more focused."
Thorpe also said he did not care what people thought of his attempts to make a comeback.
"I realised at a very early time in this that it's the worst possible career move you can make," he said.
"[But] I didn't do this for my career, I did this because I wanted to swim again.
"In the simplicity of why I'm doing it, I found the beauty in my sport that I found at a very early age. I've been able to return to that and for me that's a gift."
American Phelps, who is currently working on his own videogame, Push The Limit, welcomed the opportunity to race Thorpe.
He said: "I'm actually not too surprised, I thought he was going to probably come back earlier.
"Spending almost seven years out of the water, it's a long time, but I think he's the kind of competitor that it doesn't matter how long he spends out.
"He's tough enough and wants it enough where he can come back and get back to where he was in 2004.
"He is a great competitor, he is a hard worker, he is someone I'm happy is back in the sport. It should make a lot of events a lot more interesting."