Nico Rosberg ticked off another milestone at Silverstone on Friday, but he knows there's plenty of hard work still ahead for Mercedes.
After most of the morning session was wiped out by wet conditions, Rosberg set the fastest time of 1:32.248 in Free Practice Two as things cleared up later in the day.
The German was 0.229 quicker than Red Bull's Mark Webber with defending World Champion Sebstian Vettel while Lewis Hamilton was down in P5.
Rosberg, though, knows Mercedes still have a quite a few things to sort out before Sunday's British Grand Prix as their one-lap pace has never been a problem.
"The afternoon went well," he said. "On one lap I think the car is still quick as we have seen in the last few months so qualifying should be okay, and then the only question mark is our race speed. We practiced for the race speed today, it seemed to be okay, decent, but I'm sure it's still going to be a little bit of a problem for us.
"The next thing is that it's going to be another 10 degrees more tomorrow and Sunday, and that could completely change the picture again. That will probably make it a little bit more difficult for us so we will just have to wait and see and make the most of it."
Although he looked smooth out in practice, Rosberg still had a few problems with the balance of his car.
"I kind of went in backwards in to the high-speed corners which was not good so I still have to sort that out," he said. "The difficult thing is to understand what it is that's doing that so we have to do a lot of analysis now to try and figure it out. If you take front wing out okay it helps that area but then it makes everywhere else worse so you have understeer in the middle of corners.
"You want to do something that really just affects the entry in to high-speed corners which for example is where the car is the lowest because the faster you go the more the air pushes the car in to the ground. So it could be that the diffuser isn't working properly going in to high-speed corners because it's just stuck to the ground so it can't work properly; that's one example of things that we have to look at."
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