Five-time Tour de France winner Lance Armstrong has hinted he could retire at the end of the season.
Armstrong is wary of Ullrich ahead of the Tour
The 32-year-old Texan is targeting a record sixth win in cycling's blue-riband event.
But he told The Times newspaper: "I'm open to the possibility there will be a tap on the shoulder and someone says 'time's up'.
"This could be my last year. If I lost [The Tour] I don't know if I would say 'OK, I'm past my prime, time to go."
Despite admitting he was considering retirement, Armstrong insisted it was still only a thought.
He said: "I can't imagine being retired 12 months from now.
"Knowing myself and the people who know me best, I think they'd say the guy has to try again [if I lost the Tour]."
The US Postal rider got his season off to a solid start at the Tour of the Algarve, where he finished 34th overall after winning the individual time trial.
And he once again reiterated his belief that Germany's Jan Ullrich remained his leading contender for Tour glory.
He also admitted he would be keeping a close eye on the Telekom rider's season build-up.
"I was thinking what would I do if I heard Ullrich had won a time trial in February," he said. "I think I'd get straight down and do 50 sit-ups just to say to myself I was doing something.
"This time of the year is pure suffering for me. I'm not at my peak... but I'll be at my peak in July."