Barnes bested Bill Haas, the qualifying medalist and son of PGA Tour player Jay Haas. Mahan, the 1999 U.S. Junior Amateur Champion, defeated Dustin Bray, the ACC champion from the University of North Carolina.
Barnes and Mahan have faced each before in the U.S. Amateur Championship. In 2000 at Baltusrol, Mahan beat Barnes 3 & 2 in the second round before losing in the quarterfinals.
Barnes and Haas battled in a tight match with the highest lead belonging to Barnes at 2-up through four holes. Haas squared the match at the 15th hole and the duo halved the next two holes with pars.
At the 18th, both players missed the fairway off the tee. Barnes, with 101 yards to the uphill green, knocked his approach into a strong wind and landed his ball in a greenside bunker. Haas had 110 yards and tried to power a wedge through the wind and mis-hit the shot, advancing the ball only 50 yards into the fairway.
'I tried to smoke a sand wedge. It was awful. Terrible,' said Haas, whose father Jay watched him play Saturday after missing the cut at the Reno-Tahoe Open on Friday. 'I probably could have gotten up to one of the bunkers, but I was scared of hooking the ball left.'
Haas played his third shot into the rough and his chip stopped 15 feet from the hole. Barnes blasted his third out of the trap and it hit the stick before stopping five feet from the hole.
Haas missed his 15-footer for bogey and walked over to Barnes with his hand extended, conceding the match.
'We were on the 17th tee box and it felt like we had just started the first hole all square,' said Barnes. 'My game, my long ball got a little sketchy on the back nine. But I had been working hard on my short game and I think it showed out there for me. My short game really came through for me on the back nine.'
In the other semifinal match, Mahan took command with a birdie at the 11th to go 1-up. Mahan drained a 20-foot birdie putt at the 12th to extend the lead to 2-up but bogeyed the 13th and cut his lead to one hole.
Bray holed a six-foot par save at No. 17 to extend the match to the 18th. Like the match before them, both Mahan and Bray struggled at Oakland Hill's closing hole. Mahan chipped his fourth to a foot and tapped in to post a bogey, then watched as Bray had a chance to extend the match to extra holes.
Bray had a 10-foot par putt to keep his hopes alive but his attempt slid left, giving Mahan the 1-up win.
'I knew it's not up to me now,' said Mahan, referring to Bray's putt at 18. 'It's up to him if we're going to keep going.'
'I don't like to lose on the last hole, but it's better than getting blown out,' said Bray.
If Mahan can win the U.S. Amateur Championship Sunday, he will join Tiger Woods as the only players to win both the U.S. Junior Amateur Championship and U.S. Amateur title.