Nico Rosberg has revealed the tyre degradation suffered in Barcelona during Friday's practice is a lot worse than what was experienced in winter testing.
With the track temperature up to 42'C at the start of Friday's second session, Mercedes spent a lot of time trying to find the sweet spot with their Pirelli rubber.
Rosberg, though, was able to find some pace, clocking a 1:23.771, which put him third on the timesheets behind Jenson Button and Sebastian Vettel.
However, the German, who has spoken out in defence of Pirelli's rubber in recent days, reckons tyre management will play a big role in the outcome of this weekend's Spanish GP.
"It was very different to winter testing and a lot tougher on the tyres," Rosberg said.
"It has been a very challenging day first of all to see how to get the car right for qualifying and especially how to get it right for the race.
"I don't know where we are on tyre wear. We will have to look through the data now. Tyre management is going to be crucial here."