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.
Pos Driver Team/Car Time Gap
1. Miguel Molina Phoenix Audi 1m37.547s
2. Timo Scheider Abt Audi 1m37.596s + 0.049s
3. Augusto Farfus RBM BMW 1m37.727s + 0.180s
4. Mattias Ekstrom Abt Audi 1m38.756s + 1.209s
5. Ralf Schumacher HWA Mercedes 1m38.786s + 1.239s
6. Filipe Albuquerque Rosberg Audi 1m38.908s + 1.361s
7. Edoardo Mortara Rosberg Audi 1m39.057s + 1.510s
8. Bruno Spengler Schnitzer BMW 1m39.148s + 1.601s
9. Dirk Werner Schnitzer BMW 1m39.302s + 1.755s
10. Joey Hand RMG BMW 1m39.459s + 1.912s
11. Robert Wickens Mucke Mercedes 1m39.524s + 1.977s
12. Adrien Tambay Abt Audi 1m39.633s + 2.086s
13. Martin Tomczyk RMG BMW 1m39.698s + 2.151s
14. Roberto Merhi Persson Mercedes 1m39.772s + 2.225s
15. Andy Priaulx RBM BMW 1m39.803s + 2.256s
16. David Coulthard Mucke Mercedes 1m39.881s + 2.334s
17. Gary Paffett HWA Mercedes 1m39.898s + 2.351s
18. Mike Rockenfeller Phoenix Audi 1m39.922s + 2.375s
19. Jamie Green HWA Mercedes 1m40.056s + 2.509s
20. Christian Vietoris HWA Mercedes 1m40.200s + 2.653s
21. Rahel Frey Abt Audi 1m40.220s + 2.673s
22. Susie Wolff Persson Mercedes 1m40.424s + 2.877s