Mercedes had heads turning in Barcelona as the final days of winter testing before the 2019 season began.
The reigning champions, reportedly outpaced so far by Ferrari and Red Bull-Honda, emerged on track with a major aerodynamic upgrade for its new car.
Sources said the team has been working on the changes - including a new nose and front wing - since January, but it comes after speculation Mercedes was contemplating a wholesale re-think of its concept for the new regulations.
"We wanted to get our Melbourne specification ready as late as possible, but still early enough to test it," a source told Auto Motor und Sport.
"Compared to the presentation model, every aerodynamic surface has changed."
Lewis Hamilton said: "It was an improvement from last week which is a good step."
The Ferrari and Red Bull-Honda cars did not feature such radical bodywork changes as the test resumed at the Circuit de Catalunya on Tuesday.
Less clear is the pecking order below the top three teams.
"Sooner or later, the truth will come to light," Renault driver Nico Hulkenberg said. "I have never been good at reading coffee grounds."
However, it is rumoured Renault is concerned that Alfa Romeo, the closely Ferrari-aligned team formerly known as Sauber, may have overtaken as the fourth force.
"Alfa Romeo looks strong, but also Haas and Racing Point," Hulkenberg said.
He said Renault has definitely improved its engine compared to 2018: "Even with the new rear wing, the extra power is noticeable," said the German.
On Tuesday, it was McLaren's Lando Norris with the fastest time.
"I hope that some of those who did fast laps today are not able to go much faster," Haas driver Kevin Magnussen told Ekstra Bladet newspaper.
"Otherwise they are really fast. But right now I'm not so worried.
"Last year it was a little clearer where everyone was, but I think a better picture will emerge over the next few days," the Dane added.
