Koivun fired a 10-under-par 61 on the way to taking the lead at Blaine on Saturday (Sunday AEST).
"It felt great," Koivun said. "It's what you work for. I definitely felt some nerves."
Koivun, in his just third tournament as a professional, moved to 20-under 193 and holds a three-shot lead on Ben Kohles and Argentina's Emiliano Grillo going to Sunday's (Monday AEST) final round at TPC Twin Cities.
Koivun, who announced last month that he would forgo his senior collegiate season at Auburn to turn pro, finished on a tear with two eagles on the backside that allowed him to play the final 10 holes in 9-under.
That contributed to a tournament-record 28 on the backside, rolling in a 14-foot eagle putt on the last hole.
"I got a good break on (No.9) and kind of hit the ground running with that," Koivun said.
Koivun was dialled in from the fairway on the final hole, with his approach shot on the mark to set up his final eagle.
"I had a pretty good number (into the green)," Koivun said.
Koivun's bogey-free round included an eagle 3 on No.12.
Grillo and Kohles also were without bogeys on the way to posting 65s.
Grillo eagled No.18 shortly after Koivun accomplished that. Kohles, the first-round leader, had three birdies on both sides.
"That was my highlight of the day there on 18," Grillo said.
"I hit it well, playing from the fairway like I wanted to out here and then gave myself a lot of opportunities, couldn't take advantage of them."
Koivun said he knows there's a huge test ahead as he aims for his first professional victory.
"I'm sure some of the pins will be tucked and we've got a lot of great players behind me, but just going to stick to my game plan and try to smile," Koivun said.
Michael Brennan (63) and Chandler Phillips (66) are tied for fourth place at 16 under.
No Australians made the cut.
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