The German said his squad had anticipated a pace deficit to Sebastian Vettel and Red Bull, but was taken by surprise by Ferrari's race performance.
"We expected to be a little slower than Red Bull, but we didn't expect Ferrari to be quicker than us," Rosberg said.
"That was a bit of a surprise.
"But we're going in the right direction. Solid is maybe the best way to describe it; decent, but didn't have the pace to go for the win today."
Rosberg believes he had the fundamental pace to beat Lewis Hamilton to third, but said his race had ultimately been compromised when he lost out to the Briton in qualifying.
"For me personally I had the race pace to do a bit better than [fourth]," he said.
"I lost out a bit in qualifying yesterday and today with strategy a bit.
"It's always different when qualifying doesn't go well, [you're] always on the back foot a bit.
"I was giving everything I had, but it wasn't perfect - for me that would have been third.
"All in all it was OK though – still lots of points for the team, and we still have opportunities to get some great results."
- Sports & Recreation
- Motor Racing
- Sebastian Vettel