The 158.5km stage includes four small climbs, with the last one – the Alto El Vivero - ending just 14.3km from the finish. The climb is only five kilometres long but has a gradient of 8.5%. Sky's Chris Froome could decide to attack hard, as he did on the Peña Cabarga on Wednesday, and try and gain the few seconds he needs to take the red leader's jersey.
With time bonuses of 20-12-8 seconds awarded to the first three riders over the line, and 6-4-2 available at intermediate sprints, every second could count on the way to Madrid. Cobo currently leads Froome by just 13 seconds.
"The time bonuses could have a big say in the last three days, because there are some even in Madrid and so could decide everything," Cobo said after the finish in Noja on the spectacular northern coast, near Santander.
"We'll try and defend things like we did today by letting a break go. But I'm sure Sky will try something tomorrow and we'll have to defend my lead."
Cobo finished safely in the peloton on the stage, 7:42 behind stage winner Francesco Gavazzi (Lampre-ISD).
"The stage wasn't that difficult. The team was good, with Mathias Brandle and Dimitry Kozontchuk doing a great job at keeping the break between 10 and 11 minutes. That allowed the team to rest up and save some energy for tomorrow."