In the other semifinal, Italy's Edoardo Molinari built a 5-up lead through 11 holes and went on to defeat Austin Eaton III 2 and 1 to reach Sunday's 36-hole match play final.
Dougherty, a senior at Northwestern, rallied from an early two-hole deficit, and evened the match with a par on the eighth. He went 1-up with another par at No. 9. He moved to 2-up with a birdie at the 10th, but Deacon, a fifth-year senior at UNLV, rallied and tied the match with a par at the 14th.
Dougherty's drive at No. 15 sailed wide and he was forced to punch out into the fairway. Deacon capitalized, hitting his approach to 40 feet and lagging his putt within gimmie range. Dougherty chipped to 15 feet, but he fell one hole down.
After trading pars at No. 16, Merion's historic ``Quarry Hole,'' Dougherty charged back. He missed the green to the left on the 246-yard, par-3 hole, but rebounded by chipping in from 30 yards for a birdie to square the match.
His approach from 220 yards at the 18th hit the broadcast booth short and right of the green, leaving him a 47-yard pitch. Dougherty got a free drop and made it count, with his chip bouncing once, hitting the flagstick and stopping about a foot from the hole.
Deacon conceded the putt, and studied his options for his third shot, a chip from just short of the green. His pitch ran 10 feet past, and he missed the comebacker, giving Dougherty the victory.
The 24-year-old Molinari nearly let a large lead slip away, racing to a 5-up advantage early in the back nine. He needed what he described as the shot of his life at the 17th to close out Eaton, a 36-year-old homebuilder who won the 2004 Mid-Amateur title.
Molinari, who chipped in from a sand trap on the final hole of stroke play to get into a playoff for the match play field of 64, hit a 2-iron 15 feet past the hole and moved on with a par.
By reaching the match play final, Dougherty and Molinari receive invitations to next year's Masters and U.S. Open. Sunday's winner also advances to the 2006 British Open.