Dunlop began to turn the screw on the third and final lap and was only 0.886s down on Harrison at Glen Helen.
The Ballymoney man then took the lead of the race for the first time at Ballaugh by 0.104s from Harrison, with James Hillier third on the Bournemouth Kawasaki, 36.8s further behind.
Dunlop had the scent of victory in his nostrils and at Ramsey he was 3.4s clear.
There was no stopping the 36-year-old, who went on to secure his seventh successive Supersport win and his first on a Ducati, who has now won TT races for deven different manufacturers (Honda, Suzuki, Yamaha, Kawasaki, BMW, Paton and Ducati).
Dunlop set the fastest lap of the race at 130.313mph.
Behind Harrison, Hillier rounded out the rostrum places in third.
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