Guy Martin pipped Michael Dunlop by 0.09 seconds in the Senior race at the Southern 100 on Wednesday night.
The Tyco BMW rider led from start to finish but had to hold off a strong challenge from the Ballymoney man.
Martin set the fastest lap of the race at 111.693mph on his final lap in perfect conditions on the 4.25-mile Billown circuit.
Mar-Train Yamaha mounted Dean Harrison completed the podium, with Manxman Dan Kneen fourth and Ivan Lintin fifth.
Meanwhile, Ballymoney rider Seamus Elliott secured the biggest win of his career in the 600cc event.
Elliott, on a Yamaha, took the flag by 0.5 seconds from Russ Mountford, with Kneen third and Lintin fourth.
Mountford led before James Cowton, who was sixth in the Senior race, hit the front, only for Dean Harrison to assume control.
Yorkshireman Harrison set the fastest lap on 107.99mph but retired while holding a lengthy lead on the last of the seven laps.
Lincolnshire rider Ivan Lintin was the Supertwins victor, ahead of Cowton and Jamie Coward.
The Lightweight TT winner's advantage at the finish was 4.6 seconds, his fastest lap 103.07.
After taking runner-up spot in the Superbikes, Dunlop said he "knows he can go a lot quicker".
"I was on my Superstock bike and it is the first outing it has had. We are going in the right direction," said Dunlop.
Michael is restricting his efforts to the big bike races this week as his shoulder is still feeling the effects of a crash in the Superbike TT in June.
William Dunlop opted to miss the meeting altogether to give his rib injuries further time to heal.
The 29-year-old won the 600cc race at Skerries on Saturday and hopes to return to action at Walderstown this weekend.