Rafael Nadal's bid for a 12th Monte Carlo Masters title was shattered by Russia's Andrey Rublev, who swept to a 6-2, 4-6, 6-2 quarter-final win with the Spaniard admitting his serve was "a disaster".
Nadal, bidding to reach the semi-finals of a Masters for the 75th time, followed world No 1 Novak Djokovic, who was beaten in the third round, out of the tournament.
"Today was one of those days that my serve was a disaster," said world No 3 Nadal.
"It's always sad to lose here. I missed an opportunity to start the clay court season in the right way. But that's it. It's not the moment to complain.
"The only thing that I can do is go to Barcelona (where he plays next week) and keep practicing, try to fix the things that didn't work well."
Despite Friday's setback, 34-year-old Nadal will still be favourite to capture a 13th Roland Garros title in June and record-setting 21st major.
"For Rafa, it must be incredibly difficult to play with this pressure of always having to win," said Rublev who also made the Miami Masters semi-finals at the start of April.
"I'm shocked to see the level which he can reach despite this pressure. It's much easier to play when you have nothing to lose."
It was only the second time in 16 appearances in Monte Carlo that Nadal had failed to make the semi-finals.
World No 8 Rublev will face unseeded Casper Ruud, who put out defending champion Fabio Fognini, for a place in the final.
The other semi-final will feature Greek fourth seed Stefanos Tsitsipas against Britain's Dan Evans, the man who shocked Djokovic.
Tsitsipas is now the highest seed left after No 2 Daniil Medvedev was forced to withdraw due to a positive Covid-19 test.
Rublev is no stranger to taking out the big names - in 2019 he stunned Roger Federer at Cincinnati in just 62 minutes.
On Friday, the 23-year-old won the first set in 38 minutes but Nadal then pocketed the last four games of the 73-minute second set to level the contest.
In the decider, Rublev held his nerve, breaking the Spaniard three times on his way to a famous win, his first in three meetings with Nadal.
"He played well, he deserved it more than me," added Nadal.
"But you can't expect win against a player like him losing my serve I don't know how many times - six, seven times? It's too much."
