The 23-year-old Scot, a surprise early leader of this year's race, fears Thursday's 12th stage, which includes the tough climb up Mont Ventoux.

Millar, riding for the French Cofidis team, won the opening short time trial around the Futuroscope theme park near Poitiers, and completed his first ride through the Pyrenees on Monday.

"We've got four really tough days after the rest day - Thursday, Friday, Saturday and Sunday - which are going to be hell, I think especially if the weather's not good," he said.

"Even in the last four days, we have two days of 250km so there's no let-up.

"What we've had in the last 10 days is easy in comparison to what we've got now and we are not even halfway through the race so it's all a bit daunting still."

Having worn the yellow jersey for three of the first four days, Millar is now second in the best young rider competition for under-25 cyclists.

He believes he can compete with the best on the Tour if he continues to improve.

"I think I could become a contender within three or four years. It depends on my climbing progress," he said.

He said taking the yellow jersey had lifted his profile in the sport, but lamented the fact that cycling is still ignored in some quarters.

"My sister said it was pretty wild in all the papers, which was quite a pleasant surprise," he said.

"I know how under-rated cycling is in the UK. It's a bit of a shame the Tour de France is the only race that they acknowledge.

"Career-wise, you can do nothing all year, but if you pull out a ride in the Tour de France, it's like you've got a contract for the next year or two years."

Millar said he was delighted by his victory, which had helped earn the respect of fellow riders.

"I was just shocked. In the first two or three minutes after I crossed the line, it hit home straight away what I had just done. It was just huge," he added.

"The general respect and acknowledgement from everyone, even all the big hitters, was really good."