Gerard Kinghan won the feature Norman Brown Memorial Cup race at Bishopscourt in Co Down after Alastair Seeley blew his chance of a hat-trick.
Seeley, who earlier dominated both Masters Superbike races on the IFS Yamaha R1, was leading the big race of the day when he crashed unhurt at the hairpin on the second lap, leaving the door wide open for Kinghan.
The Randalstown man took full advantage on his Kawasaki as he seized control, edging ahead to win by 2.6 seconds from Charles Stuart on his Yamaha R1.
Peter Moloney rounded out the top three as he secured the final rostrum place by a few tenths of a second ahead of Ryan Gibson.
Aaron Clifford was the first 600cc rider over the line in fifth followed by fellow Supersport contender Aaron Armstrong.
It had all gone so well for Seeley prior to his mishap in the headline race, with the Carrick man claiming pole position in the Superbike class.
With his Spirit Moto2 machine undergoing development work, Seeley skipped the British Supersport meeting at Cadwell Park and took up the offer to ride the IFS Yamaha on his first appearance in the Masters series since 2009.
He charged clear in race one and was never threatened as the 37-year-old wrapped up the win by 11.039 seconds from Kinghan, with Stuart taking third.
Moloney, Luke Johnston on his BMW and Gibson finalised the top six, while Masters Superbike title leader Richie Ryan was only eighth as he finished behind Declan Hoey.
Race two unfolded similarly as Seeley stamped his authority on proceedings, opening a big lead at the front and closing out a double by 11.218 seconds from Kinghan, with Stuart again earning a podium place in third, a further 2.4 seconds in arrears.
Moloney, Johnston and Gibson completed the first six again, although Ryan turned the tables on Hoey to finish seventh.
The hotly contested Supersport class produced some thrilling racing as Ross Patterson, pole man Jason Lynn and Aaron Clifford battled it out in race one.
With nothing between them, it was Patterson who prevailed at the finish by only 0.171 seconds over Lynn, with Clifford only 1.3 seconds back in third place.
Mark Conlin finished fourth ahead of Aaron Armstrong and the impressive Eunan McGlinchey.
In an equally fiercely contested second race, Patterson had to dig deep to clinch a double, with Lynn agonisingly losing out by only 0.081 seconds on the line.
Clifford was four tenths of a second back in third, with Conlin 10 seconds in arrears in fourth.
McGlinchey finished fifth ahead of Armstrong.
Young Aghadowey rider McGlinchey was a double winner in the Production Twins races, while Mark Hanna and Cathal Graham sealing a win apiece in the Supertwins races.