It was one of my worst matches on the gravel for 14 years. It is difficult to see something positive, I deserved to lose, " says 32-year-old.
" I played poorly against a good player, then you have to lose.
Fognini was world number won 25 sets in a Monte Carlo. The latest to take a set of Nadal at Monaco-the gravel was the briton Kyle Edmund 2017.
Nadal, came to Monaco after a monthly break to rest his knee that he has a problem with.
The 13th-seeded Fognini will meet the oseedade serb Dusan Lajovic in the final.