"It was definitely the hottest stage of the race so far," the American said after arriving in Marseille. "Even last month in the Dauphine wasn't this hot."
"When it's like this you have to focus on hydrating during the race and after the race. I try and super-hydrate before the race because it's so difficult to drink enough in the race."
Going into Wednesday's first rest day, Armstrong is wearing the race leader's yellow jersey and looks on course to win cycling's premier event for a fifth year in a row. But the Texan is taking nothing for granted.
"There are always surprises in the Tour," he said. "It is either through miscalculated form or bad luck but there are people who come up and people who go down. We see that every year and we've seen it this year."
"At the end of the day you still have to beat the other 197 guys. We're trying to do that and were trying to win the Tour."
Armstrong said he would be busy on Wednesday.
"Obviously I have to train and I'm going to take a big nap," he said. "I'm also going to see my family, the kids. I'm going to take it easy but two days after we have a big time trial and so I have to focus on that as well."
Friday's time trial is over 47km from Gaillac to Cap Decouverte near Toulouse.
"I've seen the course, know it pretty well and I'll see it again on the morning of the race," Armstrong said.
"I've been focusing a lot on the time trials this year with my training. I had a good time trial in the Dauphine Libere race in June."
"I think it's going to be the most important time trial I've ever done in the Tour."
Armstrong finished comfortably in the main field on stage 10 to Marseille and still leads the race by 21 seconds ahead of Alexander Vinokourov of Kazakhstan.
