World number one Andy Murray blew a 4-0 lead in the deciding set as he fell to a shock defeat by Albert Ramos-Vinolas in the Monte Carlo Masters third round.
Murray went on to lose 2-6 6-2 7-5 in only his second match back after a month out with an elbow injury.
Two breaks had the Briton in command of the third set, but Spanish 15th seed Ramos-Vinolas hit back to win seven of the last eight games.
Rafael Nadal and Novak Djokovic both reached the quarter-finals.
Murray, 29, was playing his first tournament since being beaten in the second round at Indian Wells in March.
He defeated Gilles Muller in straight sets on Wednesday, but admitted afterwards his elbow injury was still causing him some problems.
Murray began his second-round match against Muller with three double faults in the first four points of the match, and his service game was again an issue against Ramos-Vinolas.
The Scot was broken seven times as 29-year-old Ramos-Vinolas claimed a first win against a world number one.
Murray showed trademark fight in fending off three break points at 4-4 in the decider, but could not respond when the Spaniard was in the ascendancy in his next service game.
Murray briefly threatened to break back with the score at 30-30 as Ramos-Vinolas served for the match, but he dragged a forehand wide and misjudged a drop shot into the net at match point.
Marin Cilic awaits Ramos-Vinolas in the last eight after beating Tomas Berdych.
Meanwhile, Jamie Murray made a winning start in the last 16 of the men’s doubles, joining forces with Bruno Soares to beat Tommy Haas and Treat Huey 6-3 6-2.
BBC tennis correspondent Russell Fuller:
Even before this match, Murray was talking of adding an extra tournament to his schedule and heading to Budapest next week.
He needs more matches and the time to rebuild trust in his serve and elbow: the French Open begins just five weeks on Sunday.
Murray has at least had four and a half hours of competitive clay-court action this week and won’t be pressing the panic button just yet.
He reached the semi-finals in Monte Carlo last year, but struggled badly in the early rounds, and still went on to dominate the rest of the year.
Nadal dominates to reach quarter-finals
Defending championNadal registered a speedy victory over promising teenager Alexander Zverev, beating him 6-1 6-1 in one hour and eight minutes.
Zverev, who turned 20 on Thursday, smashed his racquet in half after being broken twice by Nadal in the second set.
The Spaniard, aiming for his 50th clay title, will next face Argentina’s Diego Schwartzman.
World number two Djokovic fought back from a mixed second set to beat 13th-ranked Spaniard Pablo Carreno Busta 6-2 4-6 6-4.
The Serb saved two break points at 4-4 in the final set before immediately breaking Carreno Busta to set up a quarter-final with Belgium’s David Goffin.
However, Switzerland’s 2015 French Open champion Stan Wawrinka was beaten 6-4 6-4 by Uruguay’s Pablo Cuevas.