The Mercedes driver was fastest in the morning in damp conditions, over four-tenths clear of team-mate Lewis Hamilton, and was almost a quarter-of-a-second quicker than Sebastian Vettel in the far wetter afternoon session.
While Rosberg was unsure whether Red Bull showed its real potential, he believes wet conditions offer him his best chance of a good result.
"In general, it felt good and it was a good day in the wet," said Rosberg.
"Of course, you don't know where the others were. You would expect the Red Bull to be quicker anyway but I don't know how much fuel they were running.
"In general, it looked like good pace so for me it can stay wet this weekend.
"I don't know [if pole in the wet is possible] as it's too early to say but I think I've got a good shot at being right at the front in qualifying."
Vettel doubted that Friday's results would have much relevance with conditions potentially changeable all weekend.
"Obviously you can tell that people did not run a lot this afternoon because they wanted to save their tyres, as we only have one set free to use today," said the Red Bull driver.
"But come Sunday, or Saturday, you need to have the tyres and you need to make the right call still.
"It was good to get a bit of a baseline in these conditions, but you have to be sharp tomorrow or Sunday."
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