Third into the 3.21-mile stage just outside Leon, the Norwegian was nine tenths of a second quicker than his nearest rival Mikko Hirvonen. WRC leader Sebastien Loeb was third quickest.
Solberg said: "I was pushing hard, but the key was not to make a mistake. The time was quite good."
Solberg had expected his Ford team-mate Jari-Matti Latvala to beat him in the stage, but the Finn could only manage fourth fastest, 2.371sec down.
"I don't know what happened," said a mystified Latvala. "I just don't know, I have no answers for this, I honestly don't understand."
Both Hirvonen and Loeb admitted to mistakes on the stage, which was run in bright sunshine. Hirvonen said: "I made one mistake, but when you do that at this altitude then you know you are going to pay for it. I saw Seb [Loeb] have a problem on the line, so I thought I would be able to take him here."
Hirvonen was a second up on his fellow Citroen DS3 WRC driver. Loeb added: "There were one or two little mistakes, nothing major. I was sliding too much in some corners. Generally the run was good, but not great."
Evgeny Novikov was fifth, two tenths up on fellow Ford driver Mads Ostberg while Thierry Neuville made a strong competitive World Rally Car debut in his DS3 WRC, just three hundredths behind Ostberg.
Armindo Araujo suffered a troubled run and, along with fellow WRC Team Mini Portugal driver Paulo Nobre, is likely to open the road tomorrow. Araujo's Mini suffered turbo and throttle problems.
Monster World Rally Team driver Chris Atkinson was 10th fastest on his first time out with the team, while his team-mate Ken Block was one place behind after a heavy landing in the sister Fiesta.