
The number 125 wildcard turned the odds on his head with his defeats of the experienced Stepanek, 32nd in the world.
He faces off on Saturday against Dmitry Tursunov, who beat Fabrice Santoro 6-3, 7-6 (7-4)
The final on the eve of Monday's start of the Australian Open will be only the second for Guccione, whose huge left-handed serve should have made him more of a threat during his early career.
The flame-haired battler played an ATP final a year ago last weekend, losing in Adelaide to Novak Djokovic.
On his way to the Sydney showdown, "Gooch" had beaten celebrated compatriot and Davis Cup teammate Lleyton Hewitt and Czech number 14 Tomas Berdych.
One of his bullet serves apparently broke a string in Stepanek's racket, with the Czech going to the sidelines for a replacement weapon.
Guccione becomes the first wildcard to reach the Sydney final since Australian Jason Stoltenberg eight years ago.
At the end of a long battle Stepanek blinked first, double- faulting to handover a match point in Guccione's road to victory.
The Australian will begin his Aussie Open campaign against South Korean Lee Hyung-Taik.
Guccione is making his fourth Sydney appearance and improved to 7-3, having stalled in the second round in three previous appearances.