Both players overcame seeded opponents to book a place in Sunday's showdown at the ATP World Tour 250 event in Gstaad, with Monaco seeing off last year's runner-up Robin Haase, while Andujar accounted for Fernando Verdasco.
Monaco, who will be back in the top 100 in the world rankings on Monday as a result of his exploits this week, prevailed 6-3 7-6 (7-4), winning four of the 10 break points he manufactured and saving three against his own serve.
The unseeded 30-year-old, currently ranked 105 in the world, made a slow start after play was delayed by three hours due to heavy rain as he lost the first three games, but battled back in impressive fashion by breaking three times to wrap up the first set in just 36 minutes.
It was a similar story at the start of the second set as seventh seed Haase, who beat top seed and defending champion Mikhail Youzhny in the last round, seized the upper hand by breaking in the first service game.
But the gutsy Monaco levelled at 2-2 and saved break points in the seventh and ninth games to thwart Haase.
He then stood up to the test in a tie-break, claiming the initiative with the first mini-break and adding another two to seal his place in the final.
Andujar also upset the odds to progressed to the showpiece at the expense of fellow Spaniard Verdasco as he won 6-3 6-7 (2-7) 6-1.
The world number 71 has not won a title since his triumph in Casablanca in 2012, also on clay, but it was difficult to understand why as he ousted fourth seed Verdasco in two hours and 17 minutes.
Andujar failed to register an ace in the match compared to Verdasco's seven, but he saved eight break points and won seven of the 12 that he conjured up to brush aside a player ranked 33 places above him in the world with a ruthless final-set display.
It was the underdog who took the initiative in the first set with two breaks and he looked on course for a straightforward triumph when he led 5-3 in the second, only to fluff his lines and drop serve twice in as many attempts.
That left Verdasco with a chance to serve for the set but he too choked, taking the set into a tie-break in which Andujar won just two points.
Andujar hit back at the start of the decider with an immediate break and claimed another two breaks to coast into the final, taking the third match point that came his way.