"Bradley (Wiggins) is the favourite, he's the world's best stage racer now," the Garmin-Sharp veteran told Reuters as he waited outside his team bus in warm weather before the start of the first stage in Naples.
But Millar, who led the Giro during the first week of the 2011 race and was Wiggins's team mate in 2009, said he believes Sky will have at least as tough a time trying to win the Giro as they did in their successful Tour de France bid with Wiggins last July.
Wiggins won the Tour thanks to his impressive time-trialling skills and his team's ability to nullify their rivals' attacks in the mountains.
The Giro could be even harder to control.
"It's a very different kettle of fish racing here in the Giro, they (Sky) will need their A team here on top form to keep wearing the opposition down," Millar said.
Among Wiggins's main rivals should be Italian Vincenzo Nibali, who won the Tirreno-Adriatico week-long race last month in very aggressive fashion.
"Vincenzo Nibali is very motivated, he's on home terrain and he has beaten Sky twice already this year," Millar said, referring to Nibali's wins in Tirreno and in the Giro del Trentino.
While next Saturday's long individual time-trial from Saltara to Gabicce Mare is expected to favour Wiggins, Millar warned that for his team leader, defending champion Ryder Hesjedal of Canada, the route was even better.
"It's a route which has got Ryder all over it," said Millar, who with Americans Tom Danielson and Christian Vande Velde forms the old guard of Gamrin-Sharp in the Giro.
"He's probably in better shape than he was last year, it's going to be a great race."
Millar was the only rider from the 207 starters not to take part in Friday's pre-race presentation, after his wife gave birth to their second child, a boy, this week.
"It was pretty straightforward, I contacted (cycling's governing body) the UCI and did all my (obligatory pre-race) blood tests in Girona (Spain) on Thursday and here again this morning, so it was good they were so helpful," he explained.
"It was perfect, although it doesn't feel like I'm at a Grand Tour race, though I guess I'll get there soon."
Looking ahead at Sunday's team time trial, the first important challenge for the overall contenders which Garmin won in the 2008 Giro, Millar said: "It will set the tone for the race, even if you don't win.
"If you have a good day and you're smooth and fast as a squad, then it helps boost everybody's confidence. And if you have a bad one, you get on the bad foot and the morale goes down.
"It may not make a huge difference overall but it can have a major psychological effect."
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