Lance Armstrong's countdown toward retirement will reach a significant marker Sunday when he begins his last race outside the United States.

The seven-time Tour de France winner will ride in a criterium prologue to the Tour Down Under, and when the race ends seven days later and after 750 km (500 miles) around South Australia, the end of his celebrated career will loom larger.

Reflecting upon his career Saturday, the 39-year-old Armstrong acknowledged the comeback he began in the same race three years ago had not lived up to his expectations "but that's what happens in sport."

When Armstrong ended a two-year retirement in the 2009 Tour Down Under it was in the hope, even the expectation, that it would lead to his eighth win in the Tour de France.

"I thought I'd win another tour, I really did," Armstrong said.

"It was different than I expected - that's just the reality, I'm not going to make any excuses. I did everything I could ... no regrets though, none at all."

While he didn't achieve that primary goal of another Tour de France win, his comeback had succeeded in boosting the profile of his nonprofit Livestrong Foundation which raises money for cancer research and treatment.

"There's been a lot of great experiences, a lot of great travel, I've seen a lot of great things," Armstrong said.

"Without the sport the foundation and its message wouldn't be what it is today around the world, so that's probably the most important thing."

Armstrong's name has been linked to a U.S. government inquiry into drug use by cyclists, prompted in part by allegations made by his disgraced former teammate Floyd Landis.

He acknowledged he had hired a legal and communications strategist but claimed to be sanguine about the inquiry.

"I never lose sleep_- ever," he said. "It has no effect on my life, zero, that's for other people to deal with."

Armstrong said the doping shadow which hangs over the sport was among the major "cons" of his career.

"Our sport could be better organized, could be more unified and we probably need a deeper reservoir of stars because when you only have a couple that really stand out then those - for better or worse - tend to get the bulk of the attention and the bulk of the criticism and I suppose I've been in the crosshairs of that plenty," Armstrong said.

"And that drags on you after a while and it gets old but it's still a great sport, one that I'd like to see more unified and ultimately excel."

Despite the drug probe and his testy relationship with some of those in authority, Lance Armstrong's popularity remains undiminished. When he used his Twitter feed to invite cyclists to join him on an informal ride in Adelaide on Saturday morning, more than 10,000 turned out.