Fabio Fognini ended Rafael Nadal's 18-match unbeaten run at the Monte Carlo Masters with a sensational 6-4 6-2 win in their semi-final on Saturday.

Fognini had lost the previous six meetings with the second seed but caused an astonishing upset, in the process ending Nadal's seemingly inevitable march towards a 12th title in the event.

The result - achieved after one hour and 36 minutes on court - sees the Italian through to his first ATP Masters 1000 final, where he will take on Dusan Lajovic.

Nadal had not lost in Monte Carlo since 2015 but there had been worrying signs for the Spaniard in his victory over Guido Pella on Friday, as he had to rally from 4-1 down before crucially breaking serve when trailing 6-5 in the first set.

However, this time he was unable to find a way to avoid dropping a set for the first time in Monte Carlo since his second-round clash with Kyle Edmund two years ago.

Fognini recorded a break in the opening game of the contest and while Nadal immediately hit back, the former forged into a 5-4 lead to earn a chance to serve out the set.

He seized the opportunity, albeit only after a fortunate flick off the net put him 40-30 up, as a confident volley plunged Nadal into unfamiliar territory.

Few, though, would have expected the second-set procession that followed. 

Fognini reeled off five successive games in a hurry, though his charge stalled with the finishing line in sight, three match-point opportunities going begging as a break kept Nadal alive.

The left-hander held in the next game to put the pressure back on his rival, but Fognini delivered on serve at the second attempt, becoming just the fourth player to beat the 'King of Clay' in the principality.

After a rare setback on clay, an honest Nadal praised Fognini while admitting "everything" had gone wrong for him.

"The vision was clear, but I played a very bad match against a good player, so in that situation, you have to lose," he told the media

"That's what I did this afternoon. It was a tough day and he was a difficult opponent.

"It was this kind of day that everything went wrong. I probably played one of the worst matches on clay in 14 years. Today I deserved to lose because I played against a player that was better than me."

Nadal was returning to action on the ATP Tour in Monte Carlo after having to withdraw from his semi-final against Roger Federer in Indian Wells midway through March due to a knee issue.

While disappointed to lose to Fognini, the left-hander acknowledged injury problems take a toll, both mentally and physically.

"I am coming from low moments in terms of injuries, and in terms of the mental side it has not been easy to accept all the things that have been going on," he added.

"The problem is that sometimes we are used to being very, very solid all the time, and even coming back from injuries, winning and winning and winning, and things are not that easy.

"It is more difficult to hold the same level every single day when you don't have confidence in tournaments in a row or matches in a row. And that's what I needed."