MacManus had taken a one stroke overnight lead into the final day after rounds of 73-67-66 and carded a final round of 72, to finish three strokes behind eventual winner, American Harris English, who sizzled home with a final round 6-under-par, 65, to finish at 9-under-par to secure the championship title.
English had earlier rounds of 69-68-73, to give the world number 6 his first main US amateur title for 2011 after being Medalist at the 2010 US Public Links qualification section.
McManus finished with Americans, Bobby Wyatt and Ricky Werenski, at 6-under-par for his best finish this season in the US with rounds of 73-67-66-72. Despite leading the tournament following the third round, the Australian was generally pleased with his result.
"I was happy with the way I was able to come back after my opening round," said MacManus.
"It's always good to play back into contention against a field like this and overall I'm pleased with the way this tournament played out. Obviously, I would have liked to have taken the title, but overall the result is positive."
The finish at the Southern Amateur was MacManus' second top-10 this season after finishing in a tie for 7th at the NCAA D1 Southwest Regionals.
Ranked 56th in the world, MacManus was runner-up at the 2010 Asian Amateur in a prelude before claiming his biggest title at Royal Melbourne last January, winning the Master of the Amateurs with a three stroke victory over Ryan McCarthy, New Zealand's Ryan Fox and England's Andrew Sullivan.
New Zealand's Ryan Fox, who was leading in Florida following the second round, finished with a creditable top-10 result in a tie for 8th at 3-under-par with rounds of 70-66-75-70. Daniel Nisbet fired rounds of 70-72-72-69 to finish in a tie for 16th position at 1-under.
Another talented New Zealander, Ben Campbell, finished in a tie for 18th position after carding rounds of 71-73-72-68 to be at even par, along with Todd Sinnott who secured his third straight top-20 finish in a stellar run of form here in the US after rounds of 71-70-73-70.
This is Sinnott's maiden US tour and the former Victorian Boy's champion continues to impress with consistent and solid rounds against strong fields at the Dogwood, the Players Amateur and then again this week in Florida.
With results of this nature Sinnott is sure to be getting the eye of Golf Australia selectors as he continues to play stellar golf moving his world ranking from outside the top 300 before arriving in the US, to be currently 248th in the world.
Cameron Smith, the Australian Amateur Stroke Play champion, fired rounds of 71-69-74-74 to finish in a tie for 29th position at 4-over-par, with Kalem Richardson the next best of the Australians finishing at 5-over-par and in a tie for 35th position with rounds of 73-70-73-73.
Matthew Stieger fired an impressive 2-under-par during the final round with the current Australian Amateur champion finishing in a tie for 53rd position after earlier rounds of 73-74-79. Nathan Holman had opened well in Florida was the last of the Aussies who made the cut finishing in 72nd position with rounds of 70-77-74-80.
Six Australians missed the cut with Ryan McCarthy firing rounds of 81-69 to be at 8-over-par, two strokes short of making the final rounds, along with three other Aussies on the 8-over-par mark including, Maverick Antcliff with rounds of 77-73, Tim Hart with rounds of 72-78 and Rory Bourke with scores of 77-74.
Daniel Bringolf carded rounds of 78-74 and Brett Drewitt had rounds of 73-82, to finish well down in the field.
Previous champions of the Southern Amateur include Bobby Jones (3 times), Freddie Haas, Gay Brewer, Billy Joe Patton, Hubert Green (twice), Vinny Giles (twice), Lanny Wadkins (twice), Ben Crenshaw (twice), Bob Tway, Mark Brooks, Steve Lowery, Justin Leonard (twice), emerging PGA Tour players Webb Simpson (twice), Kyle Stanley (twice) and in 2004 Australian Michael Sim was victorious in Tennessee.
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