Fognini won three games in a row and held off Lajovic to seal the opening set with a stunning backhand victor down the line. Even when he was tested on Court Rainier III by Guido Pella in the quarterfinal, Nadal found dug deep to find another gear and won in straight sets.
"It was this kind of day that everything went wrong".
Whoever wins Sunday's final it will be a 22nd different victor on the men's side from the 23 ATP Tour events in 2019.
Yes, he's got more experience of the bigger stage, and yes, he's got a higher peak level, but he's also got a much worse lower level, and if you're looking at which player is guaranteed to give his all this afternoon, it's got to be Lajovic - who, let's remember, hasn't dropped a set for the entire tournament.
"I played an incredible match", Fognini said.
Fognini becomes the first Italian to win a Masters 1000 singles title and is the oldest Monte Carlo champion in the open era.
"When that happens, you have to lose".
The 11-time champion's loss on Saturday night ended a series of 25 consecutive sets won on his favourite clay as Fognini set up a final showdown against Serbian Dusan Lajovic.
Fognini trailed 3-1 early in the opening set before giving himself a mental scolding: "I told myself to calm down and play". Since then, Nadal has been in 11 more finals, winning it ten times and falling short to Novak Djokovic in 2013.
The 48th-ranked Lajovic staged an impressive comeback of his own, meanwhile, rallying from 5-1 down to beat 10th-seeded Daniil Medvedev 7-5, 6-1. In the victory, Fognini won 63 percent of his first serve points and 54 percent of his second serve points.
Fognini hopes to win his first title since last years Los Cabos Open and the ninth title of his career.
The 28-year-old Lajovic was ill before the tournament and said he needed antibiotics to recover.
But according to Fognini, he had every confidence that he could shock Nadal. But the world No. 2 broke back and held serve, before Fognini served out the match on his second attempt. "I'm still unaware of my achievement in Monte Carlo".
Playing spectacular tennis, Fognini was in full flow and he hit a delightful two-handed, backhand cross-court victor to break at the start of the second set.
"So I knew I had nothing to lose, because of course, especially when you're with him on clay, it's always really tough".