The Spaniard, who endured a miserable 2015, showed he could be returning to his best with his most impressive performance of the week, winning 6-3, 6-4 in little over an hour.
“To be back in a final is always a great feeling, especially the first week of the season,” said the Spaniard.
“I finished last year playing better and started again.”
Nadal’s serve dominated throughout against a player who troubled the best all week and knocked out the reigning champion and number four seed David Ferrer in round one.
Nadal won 100 per cent of all his first service points in the first set, 89 per cent overall and gave up only one break point, which he held. He also fired down six aces and hit 16 winners.
Nadal, the number two seed in Doha, has set up the enticing prospect of meeting Novak Djokovic in the final.
The world number one plays Tomas Berdych in the other semi-final later on Friday.
Djokovic, the number one seed, is the overwhelming favourite having won 21 of his previous 23 matches against the Czech and neither of those two losses were on a hard court as in Doha.
Marchenko, the world number 94, was by no means disgraced in his first ever match against Nadal.
Prior to this tournament, he had never taken a set off a top 10 player but as well as beating Ferrer, the amiable Ukrainian also saw off France’s Jeremy Chardy and Russia’s Teymuraz Gabashvili, both well above him in the rankings.
He may be consoled by one of the biggest pay days of his career, $57,380 (52,715 euros).