Playing in his second tour-level semi-final of the year, the 19-year-old Fonseca hammered down eight aces and struck 39 winners during the contest.
"I knew that it was going to be a tough match, Jaume is playing (well) on this surface," said world number 46 Fonseca. "He's putting a lot of pressure and you need to defend a lot of the time.
"I don't like playing against those guys because I like to play aggressive and go for winners.
"I'm happy with how I stayed mentally after an early break in the second set. I'm very happy, so let's stay hyped for the final."
The opening set was closely contested, with little to separate the pair before Fonseca edged ahead in the tiebreak, clinching a 4-3 lead and eventually taking the set.
Munar, 28, began the second set brightly, racing to a 4-2 advantage, but Fonseca showed resilience to level the score at 4-4. The youngster then claimed three of the next four games to seal the victory and advance to the final.
Fonseca, who is chasing his second tour-level title after lifting his maiden trophy in Buenos Aires in February, could break into the Top-30 for the first time if he defeats Davidovich Fokina in Sunday's showdown.
"It's been an amazing year, starting at number 130 in the world, now 40-something and into a first ATP 500 final," Fonseca said.
"We didn't go to Asia this year, I was a little bit sick. I was working with my mentality and I'm getting the opportunities... I'm just very happy to be here."
In the second semi-final, Spain's Davidovich Fokina advanced to his fourth final of the season after Frenchman Ugo Humbert retired while trailing 7-6 (4), 3-1.