Marco Wittmann was "surprised" by the power of a Formula 1 having made his first taste of the sport thanks to Toro Rosso.
Winning the DTM title last season with BMW, Wittmann was handed the test as part of his reward.
He replaced Max Verstappen in the STR10 at the Red Bull Ring on Wednesday and covered 158 laps, the most of any driver.
His best time was a 1:10.163, which put him 0.990s short of pace-setter Nico Rosberg.
"This has been an amazing experience for me," Wittmann said.
"I was surprised at the power of the car; if I compare it to DTM, it's got quite a lot more power!
"I enjoyed the whole day, which was trouble free, so I was able to do a lot of laps. I'm happy with my day, I hope the team is too.
"I would like to thank Red Bull, Scuderia Toro Rosso and BMW for giving me this great opportunity."