World number one Andy Murray was knocked out of the Madrid Open as he lost in straight sets to Croat Borna Coric in the last 16.
Coric, 59th in the world rankings having secured his first ATP Tour title in Morocco last month, won 6-3 6-3.
The 20-year-old broke his British opponent three times in the opening set, and a further break in the second was enough to secure victory.
Coric will face 23-year-old Austrian Dominic Thiem in the quarter-finals.
Murray, 29, looked frustrated as his testing clay-court season continued in the build-up to the French Open, which begins on 28 May.
"There were a lot of things that weren't particularly good," he told BBC Sport.
"I made a lot of unforced errors and I also didn't find any way to make it a more competitive match, so that's the most disappointing thing.
"Things can change fast but you need to find exactly what it is that is going wrong and how you're going to fix that and commit to it. And if I do that, I'm sure I can turn it round."
Coric lost in the second round of qualifying to Mikhail Kukushkin and only gained a place at the tournament as a 'lucky loser' because of an injury to Richard Gasquet.
This was his second victory over Murray, having also beaten him at the Dubai Championships in 2015.
In a scrappy opening set, Coric broke to lead 3-2 but a couple of forehand errors allowed Murray to break straight back.
Murray, twice a winner in Madrid, then lost his serve once more, Coric comfortably held and Murray was unable to hold his serve to stay in the set.
His frustration boiled over in the eighth game of the second set as Coric won a long rally to break, before serving out the match.
Murray will next play in Rome, where he is the defending champion.
Analysis - 'a shock to the system'
BBC tennis correspondent Russell Fuller
This result will come as a shock to Murray's system.
He had seemingly been growing in confidence, and rediscovering his rhythm little by little as he made his way from Monte Carlo to Barcelona, but now has just one week in Rome to find the form and belief which would make him a genuine contender for the French Open.
His first serve, which has been hindered by an elbow injury, was not to blame against Coric, who played aggressively and fluently and took full advantage of Murray's error-strewn performance.
Djokovic and Nadal through, Bouchard beaten
World number two Novak Djokovic reached the quarter-finals with a 6-4 7-5 defeat of Feliciano Lopez.
Defending champion Djokovic, who recently split with his coaching staff, had few problems against the 35-year-old Spaniard.
Rafael Nadal is also through to the last eight after extending his perfect record on clay this season to 12 matches without defeat with a 6-3 6-1 destruction of Nick Kyrgios.
Belgium's David Goffin secured his spot with victory over Milos Raonic 6-4 6-2, while Kei Nishikori saw off David Ferrer 6-4 6-3.
In the women's draw, Canadian Eugenie Bouchard's fine run came to an end with a 6-4 6-0 defeat by Russia's Svetlana Kuznetsova.
Bouchard had beaten Maria Sharapova and Angelique Kebver in previous rounds, but was outplayed by eighth seed Kuznetsova.
French 14th seed Kristina Mladenovic beat Romanian Sorana Cirstea 6-4 6-4 to set up a semi-final against Kuznetsova, while Romania's third seed Simona Halep thrashed Coco Vandeweghe 6-1 6-1 and will face unseeded Latvian Anastasija Sevastova.