Joey Hand kept BMW on top of the timesheets for a third straight day of DTM testing at Hockenheim on Thursday.
The American, who has switched from the RMG team to RBM for his second year in the series, set the pace in 1m33.284s on his final lap of an afternoon session that began wet and dried out as time wore on.
Track conditions were wet from the start of running, with the morning times 15 seconds slower than those managed later on.
With few cars on track at the end of the day, Hand's advantage to the second-placed Rosberg Audi of Edoardo Mortara was over a second. Mortara ended his day with a spin.
Abt Audi newcomer Jamie Green was third fastest, ahead of champion Bruno Spengler in his Schnitzer BMW.
Spengler said that the conditions had made honing his car's set-up, ahead of the season-opening round at the same track next month, difficult.
"It was an interesting day of testing, but due to the changeable conditions, executing good set-up work proved to be difficult," the Canadian said.
"On the other hand, all the three manufacturers had to cope with the same conditions.
"I'm happy with our achievements despite the adverse weather conditions and think that we made good progress. And we still have got tomorrow, haven't we?"
BMW rookie Timo Glock, eighth overall, set the pace during the morning session.
Pos Driver Car Time Gap
1. Joey Hand RBM BMW 1m33.284s
2. Edoardo Mortara Rosberg Audi 1m34.309s + 1.105s
3. Jamie Green Abt Audi 1m34.829s + 1.545s
4. Bruno Spengler Schnitzer BMW 1m35.473s + 2.189s
5. Robert Wickens HWA Mercedes 1m35.634s + 2.350s
6. Adrien Tambay Abt Audi 1m35.645s + 2.361s
7. Pascal Wehrlein RSC Mucke Mercedes 1m35.740s + 2.456s
8. Timo Glock MTEK BMW 1m35.757s + 2.473s
9. Christian Vietoris HWA Mercedes 1m35.910s + 2.626s
10. Miguel Molina Phoenix Audi 1m35.972s + 2.688s
11. Andy Priaulx RMG BMW 1m35.972s + 2.688s