MEDINAH, Ill. (AP) — Jackson Koivun crushed his tee shot on No. 18 and twirled his driver in the air as he strutted toward the fairway.

The 21-year-old Koivun looked quite comfortable in his Presidents Cup debut. More like a seasoned veteran than the youngest player on the U.S. team.

The former Auburn star defeated Si Woo Kim 2 up in singles on Sunday, part of an epic rally for the Americans in a 17-13 victory over the International squad. He went 2-0-1 in his three matches at Medinah Country Club in suburban Chicago.

“I put him in a big spot today, a really big spot,” U.S. captain Brandt Snedeker said. “That was a massive match. If we were going to have any chance today, he had to win that match. The guys were cheering in the room last night when we made that decision to put him in that spot. The whole team went crazy because they knew they wanted the kid to go up against Si Woo, and he delivered today in a big way.”

Koivun led the Tigers to the NCAA title in June. Almost two months later, the California native won the 3M Open in his third professional start, holding off Scottie Scheffler on the way to the victory.

Despite his lack of experience, Koivun was selected by Snedeker for the American team. He became the first player since Jordan Spieth in 2013 to start a year without a tour card and play in the Presidents Cup.

“I had some goals, but for it to come this quickly is definitely shocking, and I’m just thankful and grateful for the trust the captains put in me to put me out in three of the five matches,” Koivun said.

Koivun teamed with Justin Thomas for a 4-and-3 victory over Corey Conners and Nico Echavarria in fourballs on Thursday. He rested during Friday's foursomes — a 5-0 sweep for the International team — before playing with Thomas again on Saturday in a fourballs match against Min Woo Lee and Sungjae Im that was halved.

“He’s really just — he gets it. He’s very mature,” Thomas said. “It just was really cool to kind of just watch him do his thing. I’ll never in my life forget the smile he had on his face when he put the tee in the ground on Thursday morning and he was about to hit.”

Koivun fell behind at the beginning of his singles match against Kim, losing two of the first four holes. He got one back when he made an 18-footer for birdie on No. 5, a 480-yard, par-4. But Kim restored his 2-up lead when he birdied No. 9 and Koivun missed a 6-footer.

Koivun rallied with an impressive finish, winning five of the last eight holes. He tied the match with a 12-footer for birdie on No. 12. He opened a 2-up lead on No. 15, when he hit it into 7 feet and Kim missed a 6-footer.

Up one going into No. 18, Koivun showed no signs of any nerves. He blasted a 310-yard drive into the fairway and an 8-iron from 159 into 8 feet, punctuating each swing with a twirling club and a confident walk toward the green.

Koivun's victory guaranteed at least a tie for the U.S., and the Americans closed it out from there. Koivun was one of the first players to hold the Cup during the closing ceremony, capping a valuable experience for the phenom.

“Yeah, I think I’ll just take a lot of knowledge that I can be out here and I can compete,” he said. “It’s just exciting being able to call these guys my friends, and now when I go to tournaments it’s not like, ‘Oh, there’s so and so.’ It’s like, ‘I was on a team with them and we won the Presidents Cup.’”

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