The German completed 148 laps on Thursday and ended up second fastest behind Felipe Massa.
This followed two days during which Mercedes managed just 29 laps after Rosberg was sidelined by an electrical problem on Tuesday and Lewis Hamilton crashed on Wednesday following a rear braking problem.
"Today was good and made up for a lot of the loss that we had in the last two days because we got through everything and more today," said Rosberg.
"I'm sure it's going to be the same again tomorrow [when Hamilton takes over], so that will really kickstart our programme.
"It was really consistent and we got everything done that we have on the plan early. We ran out of ideas because you are limited with the tyres and in the end we were doing pitstops and things like that.
"It all worked out well with no problems whatsoever."
Rosberg added that he considers the car to be fundamentally reliable despite the issues that afflicted Mercedes on the first two days at Jerez.
He believes that the loss of track time to what he considered relatively minor problems was largely down to bad luck.
"We have just been unfortunate over the last few days, there have been problems that could very easily have been avoided but they have also been so difficult to fix.
"That is unusual with relatively small problems.
"Today, we showed that we have a very reliable car. Nothing at all went wrong today on the car, not the slightest thing.
Rosberg was positive about the handling characteristics of the car even though the team still has work to do to find a good balance.
"From lap one with this car, I feel comfortable.
"OK, there is still understeer and there's not great balance yet and there's still some optimising to do.
"But I can push straight away, feel very comfortable and I feel what's going on and that's very nice."
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