Mercedes: Ferrari progress impressive

Ferrari's Sebastian Vettel defeated Mercedes in the second grand prix of the year in Malaysia and qualified between Lewis Hamilton and Nico Rosberg on the outside of the front row in Bahrain.

Wolff said that while Ferrari's race pace improvements were clear from Melbourne, it was only this weekend that its single-lap performance had shown.

"If Sebastian hadn't been held up by Felipe [Massa], I think he would have been much closer than he was [in Australia] so that was the first indication that they were doing pretty well in the race," said Wolff.

"Since then, every race has been a step forward from Ferrari.

"What you saw first was race pace and clearly what you have seen today is qualifying pace.

"Their rate of development is very impressive.

"It has been very impressive over the winter and has been impressive over the last three races so clearly that is quite a challenge for us."

After failing to react to the Ferrari threat in Malaysia, Wolff is aware that Mercedes needs to take a more flexible approach in Sunday's Bahrain race.

"I expect the Ferraris to be strong and if the Ferraris are strong then we are going to see four cars pretty close to each other at the beginning," he said.

"That could well mean that you need to have various strategies in place to react or to be proactive or reactive.

"It depends where each of the cars is and how the race pans out."