Miguel Molina set the pace in a damp opening DTM practice session at Valencia on Friday.
The Phoenix Audi driver's 1m37.547s table-topper came late in the 90-minute session, which ended with drier conditions than the ones it started with.
Mattias Ekstrom was the first man to break the 1m40s barrier 20 minutes in, but another light rain shower prevented anybody from going faster until Bruno Spengler broke the timing beam in 1m39.148s.
Times had barely improved until beyond the hour mark, after which they began to tumble slowly but surely.
Spanish driver Molina's time was enough to edge out Timo Scheider's Abt Audi by 0.049s with Augusto Farfus the best of the BMW drivers in third for RBM.
Ekstrom, in the second Abt car, dropped to fourth by the end with HWA's Ralf Schumacher the best of the Mercedes drivers in fifth.
Rosberg Audi team-mates Filipe Albuquerque and Edoardo Mortara were next up while BMW duo Joey Hand and Dirk Werner, plus Mucke Mercedes' Robert Wickens, completed the top 10.
Championship leader Gary Paffett was only 16th quickest for HWA. His title rivals Spengler (Schnitzer BMW) and Jamie Green (HWA) were 11th and 18th.