Nico Rosberg was cautious following Friday practice for the Spanish Grand Prix, admitting that there are "a few question marks" for Mercedes.

14565 - 'A few question marks' for Rosberg

High track temperatures greeted the teams for practice, but with cooler temperatures forecast for Sunday and the operating window for the Pirelli tyres so small setups can't yet be finalised. Rosberg confirmed that the tyres could be an issue for Mercedes and that it would have to be able to react to any change in the track conditions.

"Generally all worked well today," Rosberg said. "But there are also a few question marks after the two sessions. Tyre management will be a big challenge again this weekend, and that will be the key for success. We have got plenty of options to work though this evening to find the right set-up for the conditions out there tomorrow and on Sunday."

Norbert Haug agreed that the conditions had been tricky and said the team still needed to make more progress before qualifying.

"A challenging first day with changing track conditions," Haug said. "We worked through an intensive programme with lots of laps to evaluate the two tyre compounds available. Our team learned a lot today and the plan is to make a further step tomorrow as we did today from the first to the second practice session."