The Dutchman, who earned the run in the 2010 world championship winning car for being the highest placed FR3.5 driver not connected to an F1 team, had to wait until Saturday for his first laps as his planned acclimatisation run on Friday was cancelled due to a storm at the new Moscow Raceway.
Frijns, who took to the Russian track in the Red Bull just one hour after taking pole position for Saturday's FR3.5 race, completed a handful of laps, and will do the same again on Sunday.
"It was awesome," he said. "OK, it was on demo tyres, and it had a bit of understeer, but it was a great experience."
He also admitted that he was not too disappointed to have had his F1 running cut short this weekend, as it enables him to focus more on his FR3.5 racing.
"It was a shame about the storm, but I was a bit worried about driving the F1 car too much because it could affect me when I go back to the 3.5," he added.
"I don't want to get back in my race car and start having to think about braking points and how fast I can go round certain corners because I have to adapt and get used to it again. So maybe it's not such a bad thing."