Nadal, an 11-time victor of the event, rallied from 4-1 down in the opening set before finally overhauling Argentina's Guido Pella 7-6 (7/1), 6-3.
The 13th-seeded Italian had beaten Nadal twice before on clay, and also once at the U.S. Open in five sets, but this was arguably the most impressive.
"It was this kind of day that everything went wrong".
Fognini next plays unseeded Serb Dusan Lajovic in their first-ever meeting, and one few would have predicted happening here.
"You know, I was telling him yesterday that I was knowing that I have the game to play against him".
On every one of those he was a favourite, yet lost to Aljaz Bedene, Jaume Munar, Felix Auger-Aliassime and Jiri Vesely, taking just one set in the process, and on Twitter earlier, I put a poll up asking what Fognini's SP would have been if this match took place in round one at this time last week.
Italy'sFabio Fognini shakes hands with Spain's Rafael Nadal after their semi final match. "In that situation, you have to lose", Nadal told reporters.
Fognini will face unheralded Serbian Dusan Lajovic in today's final - the first time since 2004 that neither Nadal, Djokovic nor Roger Federer have made it to the last day in the Principality. Since then, Nadal has been in 11 more finals, winning it ten times and falling short to Novak Djokovic in 2013.
Fognini hopes to win his first title since last years Los Cabos Open and the ninth title of his career.
Lajovic's resilience was highlighted when he managed to come from 1-5 down in the first set to win 7-5 after a shock collapse from the highly-rated Medvedev. In the victory, Fognini won 63 percent of his first serve points and 54 percent of his second serve points.
The 31-year-old Italian can lose his temper on court, and it got the better of him during the fourth game.
Serving for the set, Fognini saved a break point with a forehand victor down the line, and then clinched it with an equally good backhand. "I'm still unaware of reaching the final, but I'm enjoying it", added the player who has not dropped a set all week.
"But I think the beginning was a little bit odd, because I was starting break up, then break again, then 2-1 for him, break again".
It could have been an all-Serb semifinal but Medvedev knocked out the top-ranked Djokovic in Friday's quarterfinals.
Roman Polanski sues Academy for revoking his membership
The film academy is standing behind its decision to expel director Roman Polanski , who has asked a court to reinstate him. In response to the lawsuit, an Academy spokesperson said the procedure for his expulsion was "fair and reasonable".
Mueller report forced Congress to find PCs with disc drives
Mueller also found there was insufficient evidence that the president tried to obstruct any part of his investigation or report. And he said the president's personal lawyer had requested and gotten a chance to review the report before its public release.