Andy Murray's World No 1 ranking could be under threat after yet another early exit at a big tournament.
The 29-year-old went down 6-3, 6-3 to 59th-ranked Borna Coric in a huge upset result at the Mutua Madrid Open on Thursday.
Coric, 20, broke Murray's serve three times in the first set and once more in the second to book his place in the quarter-finals.
After coming off second best in the first set, the Scot was more competitive in the second, but his frustration boiled over when he got broken in the eighth game.
Coric is currently experiencing a rich vein of form, having captured his first ATP Tour title in Morocco last month. The Croatian moves on to face 23-year-old Austrian Dominic Thiem in the quarter-finals.
As for Murray, this latest defeat comes after similar exits at the Monte Carlo Masters and the Barcelona Open, although he did reach the semi-finals at the latter.
"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."
Murray will now return to the drawing board before he returns to play in Rome, where he is the defending champion.
Local hero Rafael Nadal was also in action on in Madrid on Thursday evening and he continued his serene progress in the competition with a 6-3, 6-1 victory over Nick Kyrgios.
Australian Kyrgios was expected to provide Nadal with a stern challenge, but the match turned out very differently.
The Spaniard dominated from start to finish, and once he took the first set, Kyrgios' resistance crumbled completely in the second.
Earlier in the day, Novak Djokovic, Kei Nishikori and David Goffin all booked their places in the quarter-final line-up too.
All of Thursday's third-round results
Dominic Thiem (Austria) beat Grigor Dimitrov (Bulgaria) 4-6 6-4 7-6(9)
Pablo Cuevas (Uruguay) beat Benoit Paire (France) 7-5 0-6 6-1
Borna Coric (Croatia) beat Andy Murray (Britain) 6-3 6-3
Alexander Zverev (Germany) beat Tomas Berdych (Czech Republic) 6-4 6-4
Novak Djokovic (Serbia) beat Feliciano Lopez (Spain) 6-4 7-5
David Goffin (Belgium) beat Milos Raonic (Canada) 6-4 6-2
Kei Nishikori (Japan) beat David Ferrer (Spain) 6-4 6-3
Rafael Nadal (Spain) beat Nick Kyrgios (Australia) 6-3 6-1