Mercedes team principal Ross Brawn has admitted that he is unsure how quickly the tyres will degrade during Sunday's Spanish Grand Prix.

Nico Rosberg took pole position with teammate Lewis Hamilton making up the first row, but Brawn admits that he is still unsure how the race will play out.

"The race is the key part and that is what ultimately counts," Brawn told Autosport. "It is better if you start at the front end as we are, but having a good setup and good usage of the tyres is absolutely critical.

"We have spent a lot of time this weekend devoted to it, so I don't think we could have done any more work. However, quite frankly we won't know until tomorrow, because you don't know what shape the others are in.

"We were better today on our race sim than we were yesterday, which is good, but whether it is enough we don't know."

Championship leader Sebastian Vettel starts just behind the Mercedes drivers.