Nico Rosberg admits he underestimated Sebastian Vettel after he was beaten by the Ferrari driver in qualifying for the Bahrain Grand Prix.
The Mercedes driver, on pole at Sakhir for the past two years, was unable to match the pace of pole-sitter Lewis Hamilton, finishing half a second behind the Briton.
But Vettel managed to split the Mercedes drivers like he did in Malaysia, something that caught Rosberg by surprise.
"I didn't see Sebastian being that quick," said Rosberg.
"I didn't see myself struggling that much. I've had some great starts this weekend, so maybe I can get second."
Mercedes' non-executive chairman Niki Lauda said Vettel's form had impressed him, and conceded the German was a worry for the race.
"We had the pressure from Vettel, and it's a pity that Nico couldn't get him, from a Mercedes a point of view," said Lauda.
"Vettel did a very good job. Quite impressive."
Rosberg is still feeling confident for the race, even though he said Ferrari had better long-run pace than Mercedes on Friday.
"I have better tyres than everybody else for tomorrow's race. I have better tyres, but it's a small thing, not a big difference," he said.
"Friday they were quicker than us, we worked a lot on Friday night. It was to do with the cold conditions in the evening. We are confident we improved things. We're confident we'll be quicker.
"I'm out there to win tomorrow."