As temperatures soared on the notoriously difficult Arrate climb, the Euskaltel-Euskadi leader powered a few meters ahead of a group of 11 riders to claim victory on stage four.

Second across the line was Germany's Andreas Kloeden, with Kazakh Alexandre Vinokourov third and race leader Joaquim Rodriguez fourth, all tied on the same time.

After donning a typical Basque txapela (beret) for his win, Sanchez said he was pleased to have taken such a prestigious victory but that failing to dislodge Rodriguez was a major disappointment.

"I didn't feel good on the climb but I was a little more cunning than the rest in what is a very difficult finish," Sanchez said.

"Everybody suffered in that extreme heat, and it made what is already a very tough climb even harder.

"It was great to take that win, but overall the whole race remains completely undecided.

"It's going to be very difficult to beat Rodriguez after he defended himself so well today."

Sanchez said a second victory on the climb, locally famous as a sanctuary for the rare breed of Azpi Gorri shaggy goats, was "another highpoint" in his career.

Rodriguez said that despite holding the lead after Arrate, his chances of keeping it on Saturday's decisive individual 24-kilometer time trial were slim.

"There are a lot of real time trial specialists here and a lot of them are just a few seconds behind me overall," Rodriguez told reporters.

"I just didn't have the legs today to make an all-out attack and try to get some time on them.

"The heat was really bad, but it's the same for everybody.

"I expect that tomorrow (Friday) will be won by a breakaway, all the favorites will be pretty tired after today and want to recover as best they can for the final time trial."

The Tour of the Basque Country finishes on Saturday in Zalla.