Kevin Magnussen set the fastest time on the third day of the Jerez pre-season test while Renault-powered teams continued to suffer with technical issues.
It was the first time Magnussen had tested the McLaren this year.
The Dane, who took over from team-mate Jenson Button after the lunchtime break, was over four-tenths quicker than closest challenger Felipe Massa.
“I was pushing but it wasn’t balls to the wall. We will go quicker,” Magnussen said.
“I’m pretty happy about what we did, but I’m especially happy about the laps we got in – and the lap Button did too,” He said.
“The reliability of the car has been much better than we thought it would be.”
It was another strong showing for all Mercedes-powered cars, who filled the top four spots at the end of the day’s running.
Nico Hulkenberg, in the fifth Mercedes-engine car, was in sixth.
Red Bull’s struggles continued into Thursday as they were forced to cut short Daniel Ricciardo’s day with another technical issue.
Renault’s F1 engine chief Rob White told AUTOSPORT the issues won’t be fully fixed by the end of the Jerez test.
“We had objectives for the first test that won’t have been fully realised and therefore the step up we need for Bahrain is absolutely bigger,” he said.
Red Bull have completed 14 laps in three days of running while their expected rivals McLaren, Mercedes and Ferrari together completed 212 on Thursday alone.
Marussia entered the track for the first time in 2014 but only managed five laps at the hands of Max Chilton, recording no lap time.
Full results below