The Spaniard jumped out of the Yellow Jersey group on the picturesque slopes of the Lacets de Montvernier, putting distance between him and third place rider Alejandro Valverde.
However, the group slowly gained on Contador during the remainder of the climb, cancelling out his attack.
"This was one of the toughest days on the bike," reflected the eight-time Grand Tour winner on cylingnews.com.
"I wanted to try things and see what could be done but at the end we didn't achieve anything in particular. I dropped Valverde on Glandon, this always brings confidence, but the only thing I now focus on is to recover.
"It was a very hard stage and my attacks were more driven by the heart than the legs. I was able to observe a few things [during stage 18] and we will now see how I recover for tomorrow."
The fifth placed Contador is over six minutes behind leader Chris Froome in the general classification, but Valverde is just over two minutes ahead of him.
The two-time Le Tour winner is targeting a podium finish, but understands it is a difficult task with just two mountain-top finishes remaining.
"In order for Valverde not to be on the podium, a catastrophe must take place," he said.
"He has an incredible opportunity and just by doing things the right way it's impossible for him not to reach the podium. The sport of cycling is like this and we will have to take it day by day."