In tricky conditions, the German ‘Guest Driver' ran the entire 20 minute session on slick tyres despite a shower not long after the start.
As other came to the pits to change to wet weather rubber, Frentzen, driving the Flo Rida/Jet Travel Insurance 911, stayed on-track and once the circuit dried he vaulted to the top of the time sheets.
“I must have been the only one to run slicks for the entire session,” said Frentzen.
“I didn't change tyres as I wanted to use the session to get comfortable in the car and how it was working on the track.
“It was a little damp but then when everyone changed back off the wets to slicks. I had the advantage of warm tyres and set a good time.
“If it will be dry for qualifying I don't think it will be so easy for me. The regular drivers should then be a lot quicker, and although I am happy with the car I still don't have the same experience as the top guys.
Cousins Alex (Roma Caravans) and James Davison (Lease Express) were next quickest with defending series champion Craig Baird (Jet Travel Insurance) fourth. V8 legend mark Skaife (L'Oreal Men Expert) was eighth.
Qualifying will be run later this afternoon.