Lance Armstrong faces three major rivals if he wants to win a seventh Tour de France, says his Discovery team director Johan Bruyneel.
Armstrong is gunning to extend his Tour record to seven straight wins
Ivan Basso, Alexandre Vinokourov and Jan Ullrich are the men he has picked as pretenders to Armstrong's throne.
Ullrich is the only one of the three to have won the Tour, in 1997, while Basso and Vinokourov have each come third in the past two years.
"I think those three are the real challengers," said Bruyneel.
Team CSC rider Basso is not famed for his speed but is an impressive climber, as he demonstrated last year.
"He was the only one to stay with Lance in the mountains," Bruyneel said.
"Will he be able to maintain his condition for three weeks on the Tour? That's the question mark. But it's possible."
Vinokourov and Ullrich ride for T-Mobile, and Vinokourov came home 37 seconds ahead of Armstrong in a climbing stage of the recent Dauphine Libere race.
"Vinokourov takes advantage of every opportunity," said Bruyneel.
"I think he's become more resistant over the years, and stronger in the mountains."
Ullrich has come second in the race five times, but finished only fourth last year.
However, Bruyneel still rates him as a threat.
"He is one of the best time trialists and always gets better in the second half [of the race]," he said.
"He is definitely weaker in the mountains. But he is a tough guy who never cracks."
He also picked out Joseba Beloki and Roberto Heras as potential threats, but said he did not think they had the staying power to make a big impact on the overall standings.
Armstrong says one of his toughest opponents will be the high temperatures he expects to encounter.
"I don't like the heat and that I don't perform well in the heat. We all look to 2003 as an example of that," said the six-time champion.
"You have to drink more and you have to pay attention to your energy levels. If you miss a bottle or two on a hot day, then it could be game over."