Nico Rosberg has hailed Mercedes' efforts, saying his W04 is a "pleasure" to drive and is "closer" to the front runners.
Rosberg was in action at the Circuit de Catalunya on Thursday, his fourth day behind the wheel of Mercedes' W04.
The German completed 108 laps with a best time of 1:22.6111, which he set on lap one of a seven-lap run on the medium Pirelli tyres.
His best put him fourth on Thursday's timesheet, 0.736s off the pace, with the 27-year-old encouraged by the car's performance.
"Definitely progress from last year and for sure we are going to be closer to the top guys," he said.
"That's important because it's going in the right direction.
"The team has done a really good job over the winter and it really looks like all their hard work and effort has been worthwhile.
"It's a pleasure to drive at the moment because the balance is quite good and there are a lot of positives."
However, the one issue Rosberg concedes Mercedes still need to work at is the car's reliability.
"Of course we need to improve reliability because we are having a few issues, but it's nothing major," he told ESPNF1.
"What's also looking good is our development rate at the moment. That was our weakness last year and that's what we really have to improve and that's going in the right direction too.
"There all these factors which are going well at the moment but are not quite there yet."
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