Manager Craig Levein cast his eye over Guessan as his trial period nears an end. It remains to be seen whether Hearts decide to offer him a contract, but the Ivorian did his chances no harm when he found the net.
The 25-year-old left Norwegian club Viking last month and is available as a free agent. He has been training with the Hearts first team since last week and was given a chance to impress in the Under-20 side.
Inside the Oriam Scotland indoor astrodome, a through ball by Malaury Martin found Guessan in behind the United defence on 25 minutes. His attempted left-footed ball across goal was blocked by centre-back Luc Bollan. However, Guessan collected the loose ball and dispatched it high beyond goalkeeper Deniz Mehmet with his right foot.
Hearts were 1-0 ahead at the interval and looked in relative control until the closing stages. United drew level with ten minutes remaining through Louis Appere. He collected a long ball and drove it low past Jack Hamilton - and the visitors looked destined for a point until they were reduced to ten men.
Forward Idris Kadded appeared to throw a punch at Hearts defender Chris Hamilton and was ordered off. Hearts then made the most of their one-man advantage in the dying seconds.
Striker Aidan Keena cut in from the right to loft a shot towards the top corner of the net. Mehmet got a hand to the ball but couldn’t keep it out of the net, and Keena set off to celebrate his winner.
Hearts (4-2-3-1): J Hamilton; Godinho, Gajda, C Hamilton, Sandison; Martin, Petkov; Ritchie (Zanatta 60), Morrison, Keena; Guessan (Ward 79).
Unused subs: Mason, Grigor, McGill.