Rosberg completed only fourteen laps on the opening day when an electrical fault ended his running.
To exacerbate matters for Mercedes, a brake issue pitched Lewis Hamilton into the wall on the second day having completed only fifteen laps.
"I'm very happy with today and I think it was probably a personal record for me to do almost 150 laps," he said.
"I've never been so well prepared physically as I am this season; I feel on top form and it was no problem to complete the programme. To drive the distance from Silverstone to Spa in one day with no problems whatsoever is a fantastic achievement from the team."
Rosberg was in second place, behind Ferrari's Felipe Massa. Mercedes fitted a new front wing to Rosberg's W04 this morning and tested new parts throughout the day. Rosberg set his best time on the medium compound tyres and the German added that completing two race distances made up for the lack of initial mileage.
"It really made up for the last two days and put a smile on everybody's face," he said.
"In terms of driving the car, I have a good feeling, I can push it and feel comfortable - but obviously we still have plenty of work to do to continue improving."
Lewis Hamilton resumes his running for the final day of the test tomorrow.