Rosberg did an unprecedented 172 laps, a distance of 801 km or nearly three race distances, of the Circuit de Catalunya on Tuesday after triple champion team mate Hamilton completed 156 on the first day.
Mercedes have done considerably more mileage than any other team, with Mercedes-powered Williams closest to them on 214 laps in total.
Hamilton had been due to spend all day in the car on Wednesday, with Rosberg back for Thursday, but instead the team decided to split the duties with the German lapping in the morning and Briton in the afternoon.
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"With the unprecedented mileage levels seen thus far in Barcelona, we want to keep our boys in top shape for Melbourne," the team explained on Twitter. The season starts in Australia on March 20.
Hamilton attended a sponsor event in Barcelona on Tuesday night and said then that, while he felt good, he was also sore after his first stint in a Formula One car since last November's season-ending race.
"It's like doing a workout if you haven't been to the gym for a couple of months or something like that," motorsport.com quoted him as saying. "Then the next day you have that soreness. It's exactly the same.
"So the back of my neck, because of the G-forces we are pulling is crazy... I feel my neck a little bit and my lower back."
(Reporting by Alan Baldwin; Editing by John O'Brien)
