Ben O'Connor has been left enduring increasing pressure at La Vuelta a Espana as the Perth cyclist saw another chunk taken out of his lead during the mountainous 13th stage.

Out in front since stage 6, the Decathlon AG2R La Mondiale Team rider lost 37 seconds to his nearest rival, Slovenia's Primoz Roglic, on Wednesday.

Thursday passed without further damage but there was another big setback on Friday when Roglic slashed the Australian's lead in the overall standings by almost two minutes.

O'Connor, whose lead now stands at one minute and 21 seconds, said: "I was pretty cooked. Sad times but I'm still in (the leader's) red (jersey) so at least it's a good thing."

The Australian finished 33rd in the stage, compared to Roglic's 16th. "Tomorrow, I'll just try to do my best to recover, have another day in the red jersey and see how Sunday goes," he said.

Canadian cyclist Michael Woods clinched stage victory with a decisive late breakaway in the difficult mountains of northwestern Spain.

Woods excelled on the route from Lugo to Puerto de Ancares, securing his fourth Grand Tour stage win and his third in La Vuelta.

The leading group of five riders remained tightly packed as they approached the final, gruelling ascent. However, Woods broke free 4.5km from the finish line.

Switzerland's Mauro Schmid finished second, trying but unable to match Woods's pace in the mountains, while the local audience cheered as Spaniard Marc Soler claimed third place.

"It's been a tough race. And this is a big moment of catharsis, a big release. The pressure has been building and to get this win now, I'm on cloud nine," Woods said.

"I knew that as long as I had the guys within striking distance I had a shot at winning because the climb really suited me.

"I didn't want to go as early as Mauro (Schmid) did but then I had to. It was a long four kilometres to be alone."

The 37-year-old Canadian pointed to the red maple leaf on his Canadian champion jersey as he crossed the finish line, celebrating his hard-fought and strategic win.