Nico Rosberg says Mercedes' rivals have yet to show their true hand as the final F1 test of the year moves towards its conclusion.

Rosberg was quickest at lunch on the final day in Barcelona having posted the quickest time of the week, going even faster than Lewis Hamilton had managed on Saturday.

However the German said it was important not to get carried away with the pace shown on track at Catalunya given that others haven't shown their true speed through the testing season.

"As we all know, you have to be very careful with the results of testing because you just don't know what other people are doing," he was quoted by ESPN. "We know for a fact that our competitors haven't showed their potential by far, we know that for sure.

"What I can say is that we have made progress and I'm pleased with the way the winter has gone. Everything has gone to plan and the reliability has been sorted out. But what also needs to be said is that Melbourne will see completely different conditions again as it will be hotter.

"We know for a fact last year that one of our big weaknesses was tyre degradation. We've learnt a lot about that, but how far are we [to solving the issue], we will have to wait and see. Pleased is probably the best word [to describe my feeling], but we'll see."

The speed of the Mercedes had been a topic of discussion after Hamilton's pace-setting performance on Saturday, although Lotus team boss Eric Boullier was amongst those who were unconcerned by the fact that the F1 W04 was leading the way.

"I think they've definitely improved," he said. "But if you remember last year they were competitive at the beginning of the season as well. So there is no reason why they would not be competitive again at the beginning of the season, so let's wait and see in Melbourne.

"There is a big challenge to see what is going on because there was one-lap performance from some teams and especially the Mercedes car was fast, definitely. Then you have a different story when you go for higher fuel and the race simulation. I think the [one-lap] performance order will be different to the order in the race, so that's going to be interesting."