On a day when Carlos Alcaraz shelved his latest bid to quickly regain his world No.1 spot from Jannik Sinner, withdrawing from Barcelona with a wrist injury, de Minaur missed a chance to cash in on the Spaniard's absence as he suffered a deflating 6-4 6-3 loss to world no.88 Hamad Medjedovic.
Fresh from his promising clay-court season opener in the Monte Carlo Masters, where he had reached the quarter-finals only to lose a match he felt he should have won against Valentin Vacherot, de Minaur had explained how he was keen to kick on as third seed at the Real Club de Tenis Barcelona.
But while managing to land only 36 percent of his first deliveries and coughing up 25 unforced errors on the Rafa Nadal Court, the Spanish-based Sydneysider was nowhere near his best as he succumbed in an hour and 35 minutes to the 22-year-old Serb prospect.
There was clear frustration for the world no.7 de Minaur, third seed in Barcelona, that he couldn't find a way back into the match in the second set, especially when he held break point at 4-3 and then had an umpiring call and video review go against him just when he thought he'd broken.
In all, he won just one of nine break points and Medjedovic reeled off those final three games to earn perhaps the biggest victory of his young career, reaching his first ATP 500 quarter-final.
De Minaur has found it hard since his triumph in the Rotterdam Open in February to get any convincing run going, suffering early exits in Acapulco, Indian Wells and Miami before those much-needed back-to-back wins in Monte Carlo last week.
But his Barcelona exit felt like a missed chance with world No.2 Alcaraz, the top seed, withdrawing after suffering his wrist troubled him during his first-round win over Finland's Otto Virtanen.
Alcaraz, who lost his world No.1 spot to Jannik Sinner on Monday after his Monte Carlo final defeat by the Italian, explained: "After tests, we've realised the injury is more serious than any of us expected, and I need to listen to my body to ensure it doesn't affect me in the future.
"It is with great sadness that I have to return home to begin my recovery as soon as possible with my team, the doctors and the physio, and to be, or try to be, as fit as possible for the tournaments I have coming up."
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