The German hit the front early in the afternoon, setting a 1m 09.113s that was never headed. He also broke into triple figures for the day, managing 117 laps before the chequered flag fell.
Williams’ Valtteri Bottas had been quickest in the morning with a 1m 10.029s, but didn’t improve in the afternoon and consequently slipped to third, behind Rosberg and Ferrari’s Esteban Gutierrez.
The Mexican, enjoying only his second day of running in the SF15-T, got to within eight tenths of Rosberg’s benchmark in the final hour of the day.
"It's always nice to be able to drive the current Formula 1 car and I want to thank the team for giving me this opportunity," Gutierrez reflected. "It was a positive day, we did a good job and we completed the programme we had set ourselves, gathering plenty of data that we can now carefully analyse."
Reigning DTM champion Marco Wittmann was fourth fastest for Toro Rosso, on what was his maiden experience of F1 machinery. The 25-year-old German was the last man to get within one second of Rosberg, and also the day’s busiest pilot, racking up a mighty 158 laps.
"This has been an amazing experience for me," Wittmann said. "I was surprised at the power of the car. I enjoyed the whole day, which was trouble free, so I was able to do a lot of laps."
Having set the pace for Mercedes on Tuesday, Pascal Wehrlein wound up fifth fastest in Force India’s B-spec VJM08.
Jolyon Palmer secured sixth for Lotus in the closing minutes of the day, while Fernando Alonso was seventh on what was a largely encouraging day for McLaren. The Spaniard’s time of 1m 10.736s was faster than he managed in qualifying for the Grand Prix, and he also racked up 104 laps - the highest tally McLaren have managed over a single day all season.
Red Bull’s Daniel Ricciardo and Sauber’s Felipe Nasr rounded out the day’s runners, with Marussia having opted to skip the test.
"To get in more than 100 laps was very good," Ricciardo said. "We learned a few things and we tested a lot of stuff. We had a big aero rake on the car this morning that looked like something out of ‘The Terminator’ but it’s all good information for the aero guys and will help going forward. We have some updates coming to Silverstone as well, so I hope we can take the fight to our immediate rivals."
Ricciardo was to blame for the day’s only red flag, after his RB11 ground to a halt at the entry to the pit lane late on in the afternoon.
Unofficial Wednesday testing times from Spielberg