The West Yorkshire rider left it late to claim provisional pole on his Silicone Engineering Kawasaki with a speed of 125.438mph, knocking Manx rider Conor Cummins (Padgett’s Honda) off the top spot by just 0.398s.
Peter Hickman, who did the double in the Supersport class last year, was third fastest on the Trooper Beer Smiths Triumph at 124.834mph, one second back on Harrison.
Derek McGee, who had led the session on his B&W/Lady B Kawasaki, finished fourth fastest with a lap of 124.431mph, which left him only 0.6s back on Hickman.
Tobermore’s Adam McLean was also in contention on the McAdoo Kawasaki as he slotted into fifth on the times, clocking a speed of 123.454mph – 3.4 seconds down on Harrison.
England’s Davey Todd completed the top six on the Burrows Engineering Racing Honda (123.035mph), followed by Darren Cooper and Dan Cooper.
Magherafelt’s Paul Jordan was an early retirement on his 600 Yamaha at the beginning of the session.
Lee Johnston (Padgett’s Racing), was 12th fastest following a lap of 120.943mph.
1 Dean Harrison Kawasaki 125.438
2 Conor Cummins Honda 125.203
3 Peter Hickman Triumph 124.834
4 Derek McGee Kawasaki 124.431
5 Adam McLean Kawasaki 123.454
6 Davey Todd Honda 123.035
7 Darren Cooper Kawasaki 122.148
8 Daniel Cooper Honda 121.969
9 Joey Thompson Kawasaki 121.378
10 Michael Sweeney Yamaha 121.152