Armstrong's first Le Tour win came in 1999
Lance Armstrong believes former team-mate Alberto Contador will pose the biggest threat to his ambition of winning an eighth Tour de France title.
Reigning champion Contador looks in top form after coming second in the Criterium du Dauphine last week and winning March's Paris-Nice race.
"He goes in as the clear favourite, he'll be difficult to beat," admitted the 38-year-old RadioShack rider.
"Alberto's a complete rider with very few weaknesses."
Armstrong was Contador's team-mate at Astana last year, but the pair struggled to keep a lid on their strained relationship with the Spaniard admitting to a "tense and delicate" situation throughout the three-week Tour.
"On this Tour, the days in the hotel were harder than those on the road," said Contador last year.
"He is a great rider but it is another thing on a personal level, where I have never had great admiration for him and I never will."
The American eventually finished third, more than five minutes behind his rival and Armstrong believes the knowledge of Contador that he gleaned from that experience will give him "an advantage" in this year's race.
Armstrong is well aware of the 27-year-old's ominous display in Dauphine, which included a prologue and penultimate stage win in the mountains.
"He climbs better [than anybody else] and he time-trials with the best," said Armstrong after finishing seven seconds behind stage three winner Frank Schleck in the Tour of Switzerland - the American's final race before Le Tour.
The Tour de France begins on 3 July in the Dutch city of Rotterdam, before moving through Belgium and into France.
And Armstrong, whose last Tour win came in 2005, fears that first week could be unusually testing.
"It's very windy there, we already saw that in the Tour of Italy," he said. "Then there are the stages around Brussels and in the Ardennes, as well as the cobbled sections in the north of France.
"It's going to be a nervous and dangerous first week."
Armstrong's RadioShack team, meanwhile, were dealt an embarrassing blow when they were omitted from the entries for the Tour of Spain, which begins on 28 August.
Sixteen teams were preselected under an agreement between the race's organisers and the International Cycling Union (UCI), while another six, including Britain's Team Sky but not RadioShack, received invitations to one of the sport's three Grand Tours.
"They told us we were not selected because other teams were better in their eyes. We are not happy about it," said team spokesman Philippe Maertens.
RadioShack sports director Alain Gallopin revealed, though, that Armstrong was not on their list of 15 riders for the race "because it would have been silly to ask a 38-year-old to ride two big tours like the Tour de France and the Vuelta".
Gallopin added: "But with Andreas Kloeden, Levi Leipheimer, Janez Brajkovic, Haimar Zubeldia and Chris Horner, our team would have been very competitive."
There was better news for RadioShack on the road in Switzerland, though, as Armstrong gained ground in the general classification.
Armstrong finished 15th in the 197-km (122-mile) stage from Sierre to Schwarzenburg to move up nine places to 21st in the overall standings, 30 seconds behind leader Tony Martin, of Team HTC-Columbia.
Swiss Fabian Cancellara (Saxo Bank) is a mere second behind Germany's Martin, with Team Sky's Swedish rider Thomas Lovkvist another nine seconds back in third.
Tuesday's fourth stage takes the riders the 192km from Schwarzenburg to Wettingen. The race ends in Liestal on Sunday.