Mercedes arrived at the Spanish Grand Prix after making improvements to their W08 car and after Friday's practice sessions Lewis Hamilton and Valtteri Bottas were both impressed with the new offering.
Among the updates from the constructors' champions was a new nose design and it immediately paid dividends in FP1, as Hamilton went fastest, ahead of his Finnish team-mate.
Hamilton was quickest in the day's second session too, his advantage over Bottas widening by the slenderest of margins.
The Silver Arrows were nine tenths clear of Ferrari in FP1, but that was shaved to three as they finished day one in the ascendancy.
"First practice was very, very good but in second practice the track changed and shifted quite a lot, so it was slippery and quite a lot slower for everyone - especially with the gusts of wind," said Hamilton.
"It was massively challenging but still fun nonetheless.
"The team has done an amazing job with the upgrades and the car is working just as we expected.
"It's been a much better start to the weekend for me than in Sochi, so I'm very happy. We've just got to keep our heads down and keep working, we want to be number one."
Bottas - who won the last race in Russia - added: "Today was an important one for us with the new upgrade package on the car.
"Our target was to evaluate if the new upgrades are working well and it seems like they are. At this time it's way too early to tell exactly where we are, but everything seems to be working well so far."