The penultimate stage is a demanding mountainous route of 185.7 kilometers (115.3 miles) starting in Santo Estevo de Ribas de Sil and finishing at the special-category summit at Puerto de Ancares. Froome shaved some time off Contador's lead on the previous mountainous stage.
"We have a hard stage ahead of us," Contador said. "And we are facing a team with a great climber in Froome. So I will see how my legs respond."
Earlier on Friday, the peloton swallowed up three escapees at the foot of the stage's second category-two climbs.
Sky's Dario Cataldo crashed as the peloton accelerated down from the category-two Alto Monte Faro. After lying prone on his back on the side of the road, he was able to recover and eventually finish the stage.
Contador said he was behind Cataldo when he crashed and almost went down with him.
"It was complicated. I don't know how I managed to avoid going down," Contador said. "During the last kilometer I fell back to stay clear in case anyone else fell."
The 21-stage race finishes with an individual time trial in Santiago de Compostela on Sunday.